new autoregressive check-in

api_new_autoregressive.py

@@ -134,8 +134,8 @@ class TextToSpeech:
         self.tokenizer = VoiceBpeTokenizer()
         download_models()
 
-        self.autoregressive = AutoregressiveCodegen(512, 12).cpu().eval()
-        self.autoregressive.load_state_dict(torch.load('D:\\dlas\\experiments\\train_autoregressive_codegen\\models\\23000_codegen_ema.pth'))
+        self.autoregressive = AutoregressiveCodegen(1024, 16).cpu().eval()
+        self.autoregressive.load_state_dict(torch.load('X:\\dlas\\experiments\\train_autoregressive_codegen\\models\\11000_codegen_ema.pth'))
 
         self.clip = VoiceCLIP(dim_text=512, dim_speech=512, dim_latent=512, num_text_tokens=256, text_enc_depth=12,
                              text_seq_len=350, text_heads=8,

models/new_autoregressive.py

@@ -1,11 +1,9 @@
-import functools
-
 import torch
 import torch.nn as nn
 import torch.nn.functional as F
 from transformers import GPT2PreTrainedModel, GPT2Config
+from models.xtransformers import TransformerWrapper, Encoder, Decoder
 from transformers.modeling_outputs import CausalLMOutputWithCrossAttentions
-from x_transformers import TransformerWrapper, Encoder, Decoder
 
 from models.arch_util import AttentionBlock
 
@@ -87,8 +85,8 @@ class InferenceModel(GPT2PreTrainedModel):
         assert labels is None  # Training not supported by this inference model.
         return_dict = return_dict if return_dict is not None else self.config.use_return_dict
 
-        hidden_states = self.transformer.decoder(input_ids, context=self.context, return_embeddings=True)
-        logits = self.transformer.decoder.transformer.to_logits(hidden_states)
+        hidden_states = self.transformer.decoder(input_ids, full_context=self.context, return_embeddings=True)
+        logits = self.transformer.decoder.to_logits(hidden_states)
 
         if not return_dict:
             return (logits, )
@@ -157,54 +155,22 @@ class ConditioningEncoder(nn.Module):
         return h.mean(dim=2)
 
 
-class CheckpointedLayer(nn.Module):
-    """
-    Wraps a module. When forward() is called, passes kwargs that require_grad through torch.checkpoint() and bypasses
-    checkpoint for all other args.
-    """
-    def __init__(self, wrap):
-        super().__init__()
-        self.wrap = wrap
-
-    def forward(self, x, *args, **kwargs):
-        for k, v in kwargs.items():
-            assert not (isinstance(v, torch.Tensor) and v.requires_grad)  # This would screw up checkpointing.
-        partial = functools.partial(self.wrap, **kwargs)
-        return torch.utils.checkpoint.checkpoint(partial, x, *args)
-
-
-class CheckpointedXTransformerWrapper(nn.Module):
-    """
-    Wraps a TransformerWrapper and applies CheckpointedLayer to each layer.
-    """
-    def __init__(self, checkpoint=True, **xtransformer_kwargs):
-        super().__init__()
-        self.transformer = TransformerWrapper(**xtransformer_kwargs)
-
-        if not checkpoint:
-            return
-        for i in range(len(self.transformer.attn_layers.layers)):
-            n, b, r = self.transformer.attn_layers.layers[i]
-            self.transformer.attn_layers.layers[i] = nn.ModuleList([n, CheckpointedLayer(b), r])
-
-    def forward(self, x, **kwargs):
-        return self.transformer(x, **kwargs)
-
-
 class AutoregressiveCodegen(nn.Module):
-    def __init__(self, model_dim, depth, num_text_tokens=256, num_mel_tokens=8194, max_text_tokens=4000,
-                 max_mel_tokens=4000, dropout=.1):
+    def __init__(self, model_dim, depth, num_text_tokens=256, num_mel_tokens=8194, dropout=.1):
         super().__init__()
+        assert depth >= 8  # This is the minimum bound to support the context interleaving that happens later.
 
         self.START_TOKEN=8192
         self.STOP_TOKEN=8193
-        self.max_mel_tokens = max_mel_tokens
-        self.minicoder = ConditioningEncoder(80, model_dim, do_checkpointing=False)
-        self.encoder = CheckpointedXTransformerWrapper(
+        self.max_text_token_id = num_text_tokens
+        self.max_mel_token_id = num_mel_tokens
+        self.mel_embedding = ConditioningEncoder(80, model_dim, do_checkpointing=False)
+        self.encoder = TransformerWrapper(
                                   num_tokens=num_text_tokens,
-                                  max_seq_len=max_text_tokens,
+                                  use_pos_emb=False,
+                                  max_seq_len=-1,
                                   attn_layers = Encoder(
-                                      depth=depth//2,
+                                      depth=depth,
                                       heads=model_dim//64,
                                       dim=model_dim,
                                       attn_dropout=dropout,
@@ -213,11 +179,14 @@ class AutoregressiveCodegen(nn.Module):
                                       ff_glu=True,
                                       ff_mult=1,
                                       rotary_pos_emb=True,
-                                      rel_pos_bias=True,
+                                      attn_rel_pos_bias=True,
                                   ))
-        self.decoder = CheckpointedXTransformerWrapper(
+        self.encoder.norm = nn.Identity()  # This layer and the next are unused.
+        self.encoder.to_logits = nn.Identity()
+        self.decoder = TransformerWrapper(
                                   num_tokens=num_mel_tokens,
-                                  max_seq_len=max_mel_tokens,
+                                  use_pos_emb=False,
+                                  max_seq_len=-1,
                                   attn_layers=Decoder(
                                       depth=depth,
                                       heads=model_dim//64,
@@ -228,18 +197,21 @@ class AutoregressiveCodegen(nn.Module):
                                       ff_glu=True,
                                       ff_mult=1,
                                       rotary_pos_emb=True,
-                                      rel_pos_bias=True,
                                       cross_attend=True,
+                                      attn_rel_pos_bias=True,
                                   ))
 
     def get_grad_norm_parameter_groups(self):
         return {
             'encoder': list(self.encoder.parameters()),
             'decoder': list(self.decoder.parameters()),
-            'minicoder': list(self.minicoder.parameters()),
+            'minicoder': list(self.mel_embedding.parameters()),
         }
 
     def forward(self, text_codes, conditioning_signal, mel_codes, wav_lengths, return_loss=True):
+        assert text_codes.max() < self.max_text_token_id and text_codes.min() >= 0, f'Invalid text code encountered: {text_codes.max()}, {text_codes.min()}'
+        assert mel_codes.max() < self.max_mel_token_id and mel_codes.min() >= 0, f'Invalid mel code encountered: {mel_codes.max()}, {mel_codes.min()}'
+
         # Format mel_codes with a stop token on the end.
         mel_lengths = wav_lengths // 1024 + 1
         for b in range(mel_codes.shape[0]):
@@ -251,43 +223,51 @@ class AutoregressiveCodegen(nn.Module):
             conditioning_signal = conditioning_signal.unsqueeze(1)
         cond_embs = []
         for i in range(conditioning_signal.shape[1]):
-            cond_embs.append(self.minicoder(conditioning_signal[:, i]))
+            cond_embs.append(self.mel_embedding(conditioning_signal[:, i]))
         cond_emb = torch.stack(cond_embs, dim=1).mean(dim=1, keepdim=True)
-        enc_text = self.encoder(text_codes, return_embeddings=True)
-        context = torch.cat([cond_emb, enc_text], dim=1)
+        _, enc_text = self.encoder(text_codes, return_hiddens=True)
+        # Interleave cond_emb into the first few contexts.
+        full_context = enc_text
+        full_context[1] = cond_emb
+        full_context[3] = cond_emb
+        full_context[6] = cond_emb
 
         # Execute the decoder
         dec_inputs = F.pad(mel_codes, (1,0), value=self.START_TOKEN)[:, :-1]
-        dec = self.decoder(dec_inputs, context=context)
+        dec = self.decoder(dec_inputs, full_context=full_context)
         if not return_loss:
             return dec
         loss_mel = F.cross_entropy(dec.permute(0,2,1), mel_codes)
         return loss_mel
 
-    def generate(self, conditioning_signal, text_codes, **hf_generate_kwargs):
-        if not hasattr(self, 'inference_model'):
-            self.inference_model = InferenceModel(self)
-
+    def generate(self, conditioning_signal, text_codes, max_tokens=256, **hf_generate_kwargs):
+        inference_model = InferenceModel(self)
+        # Build the context
         if len(conditioning_signal.shape) != 4:
             conditioning_signal = conditioning_signal.unsqueeze(1)
         cond_embs = []
         for i in range(conditioning_signal.shape[1]):
-            cond_embs.append(self.minicoder(conditioning_signal[:, i]))
+            cond_embs.append(self.mel_embedding(conditioning_signal[:, i]))
         cond_emb = torch.stack(cond_embs, dim=1).mean(dim=1, keepdim=True)
-        enc_text = self.encoder(text_codes, return_embeddings=True)
-        context = torch.cat([cond_emb, enc_text], dim=1)
-        self.inference_model.store_context(context)
-
-        gen = self.inference_model.generate(bos_token_id=self.START_TOKEN, pad_token_id=self.STOP_TOKEN, eos_token_id=self.STOP_TOKEN,
-                                            max_length=250, output_attentions=False, return_dict_in_generate=True,
+        _, enc_text = self.encoder(text_codes, return_hiddens=True)
+        # Interleave cond_emb into the first few contexts.
+        full_context = enc_text
+        full_context[1] = cond_emb
+        full_context[3] = cond_emb
+        full_context[6] = cond_emb
+        inference_model.store_context(full_context)
+
+        gen = inference_model.generate(bos_token_id=self.START_TOKEN, pad_token_id=self.STOP_TOKEN, eos_token_id=self.STOP_TOKEN,
+                                            max_length=max_tokens, output_attentions=False, return_dict_in_generate=True,
                                             **hf_generate_kwargs)
         return gen.sequences
 
 
 if __name__ == '__main__':
-    codegen = AutoregressiveCodegen(1024, 20)
-    codegen.generate(torch.randn((1,80,120)), torch.randint(0,256,(1,200)))
+    codegen = AutoregressiveCodegen(256, 10)
+    torch.save(codegen.state_dict(), 'sample.pth')
+    #codegen.generate(torch.randn((1,80,120)), torch.randint(0,256,(1,200)))
     codegen(torch.randint(0,256, (2,200)),
             torch.randn(2,80,120),
             torch.randint(0,8192, (2,350)),
-            torch.tensor([192,350]))
+            torch.tensor([192,350]))

models/xtransformers.py

@@ -0,0 +1,1259 @@
+import functools
+import math
+import torch
+from torch import nn, einsum
+import torch.nn.functional as F
+from functools import partial
+from inspect import isfunction
+from collections import namedtuple
+
+from einops import rearrange, repeat, reduce
+from einops.layers.torch import Rearrange
+
+from entmax import entmax15
+from torch.utils.checkpoint import checkpoint
+
+from x_transformers.autoregressive_wrapper import AutoregressiveWrapper
+
+DEFAULT_DIM_HEAD = 64
+
+Intermediates = namedtuple('Intermediates', [
+    'pre_softmax_attn',
+    'post_softmax_attn'
+])
+
+LayerIntermediates = namedtuple('Intermediates', [
+    'hiddens',
+    'attn_intermediates'
+])
+
+
+# helpers
+
+def exists(val):
+    return val is not None
+
+
+def default(val, d):
+    if exists(val):
+        return val
+    return d() if isfunction(d) else d
+
+
+def cast_tuple(val, depth):
+    return val if isinstance(val, tuple) else (val,) * depth
+
+
+class always():
+    def __init__(self, val):
+        self.val = val
+
+    def __call__(self, *args, **kwargs):
+        return self.val
+
+
+class not_equals():
+    def __init__(self, val):
+        self.val = val
+
+    def __call__(self, x, *args, **kwargs):
+        return x != self.val
+
+
+class equals():
+    def __init__(self, val):
+        self.val = val
+
+    def __call__(self, x, *args, **kwargs):
+        return x == self.val
+
+
+def max_neg_value(tensor):
+    return -torch.finfo(tensor.dtype).max
+
+
+def l2norm(t):
+    return F.normalize(t, p=2, dim=-1)
+
+
+# init helpers
+
+def init_zero_(layer):
+    nn.init.constant_(layer.weight, 0.)
+    if exists(layer.bias):
+        nn.init.constant_(layer.bias, 0.)
+
+
+# keyword argument helpers
+
+def pick_and_pop(keys, d):
+    values = list(map(lambda key: d.pop(key), keys))
+    return dict(zip(keys, values))
+
+
+def group_dict_by_key(cond, d):
+    return_val = [dict(), dict()]
+    for key in d.keys():
+        match = bool(cond(key))
+        ind = int(not match)
+        return_val[ind][key] = d[key]
+    return (*return_val,)
+
+
+def string_begins_with(prefix, str):
+    return str.startswith(prefix)
+
+
+def group_by_key_prefix(prefix, d):
+    return group_dict_by_key(partial(string_begins_with, prefix), d)
+
+
+def groupby_prefix_and_trim(prefix, d):
+    kwargs_with_prefix, kwargs = group_dict_by_key(partial(string_begins_with, prefix), d)
+    kwargs_without_prefix = dict(map(lambda x: (x[0][len(prefix):], x[1]), tuple(kwargs_with_prefix.items())))
+    return kwargs_without_prefix, kwargs
+
+
+# activations
+
+class ReluSquared(nn.Module):
+    def forward(self, x):
+        return F.relu(x) ** 2
+
+
+# positional embeddings
+
+class AbsolutePositionalEmbedding(nn.Module):
+    def __init__(self, dim, max_seq_len):
+        super().__init__()
+        self.scale = dim ** -0.5
+        self.emb = nn.Embedding(max_seq_len, dim)
+
+    def forward(self, x):
+        n = torch.arange(x.shape[1], device=x.device)
+        pos_emb = self.emb(n)
+        pos_emb = rearrange(pos_emb, 'n d -> () n d')
+        return pos_emb * self.scale
+
+
+class FixedPositionalEmbedding(nn.Module):
+    def __init__(self, dim):
+        super().__init__()
+        inv_freq = 1. / (10000 ** (torch.arange(0, dim, 2).float() / dim))
+        self.register_buffer('inv_freq', inv_freq)
+
+    def forward(self, x, seq_dim=1, offset=0):
+        t = torch.arange(x.shape[seq_dim], device=x.device).type_as(self.inv_freq) + offset
+        sinusoid_inp = torch.einsum('i , j -> i j', t, self.inv_freq)
+        emb = torch.cat((sinusoid_inp.sin(), sinusoid_inp.cos()), dim=-1)
+        return rearrange(emb, 'n d -> () n d')
+
+
+class RelativePositionBias(nn.Module):
+    def __init__(self, scale, causal=False, num_buckets=32, max_distance=128, heads=8):
+        super().__init__()
+        self.scale = scale
+        self.causal = causal
+        self.num_buckets = num_buckets
+        self.max_distance = max_distance
+        self.relative_attention_bias = nn.Embedding(num_buckets, heads)
+
+    @staticmethod
+    def _relative_position_bucket(relative_position, causal=True, num_buckets=32, max_distance=128):
+        ret = 0
+        n = -relative_position
+        if not causal:
+            num_buckets //= 2
+            ret += (n < 0).long() * num_buckets
+            n = torch.abs(n)
+        else:
+            n = torch.max(n, torch.zeros_like(n))
+
+        max_exact = num_buckets // 2
+        is_small = n < max_exact
+
+        val_if_large = max_exact + (
+                torch.log(n.float() / max_exact) / math.log(max_distance / max_exact) * (num_buckets - max_exact)
+        ).long()
+        val_if_large = torch.min(val_if_large, torch.full_like(val_if_large, num_buckets - 1))
+
+        ret += torch.where(is_small, n, val_if_large)
+        return ret
+
+    def forward(self, qk_dots):
+        i, j, device = *qk_dots.shape[-2:], qk_dots.device
+        q_pos = torch.arange(i, dtype=torch.long, device=device)
+        k_pos = torch.arange(j, dtype=torch.long, device=device)
+        rel_pos = k_pos[None, :] - q_pos[:, None]
+        rp_bucket = self._relative_position_bucket(rel_pos, causal=self.causal, num_buckets=self.num_buckets,
+                                                   max_distance=self.max_distance)
+        values = self.relative_attention_bias(rp_bucket)
+        bias = rearrange(values, 'i j h -> () h i j')
+        return qk_dots + (bias * self.scale)
+
+
+class AlibiPositionalBias(nn.Module):
+    def __init__(self, heads, **kwargs):
+        super().__init__()
+        self.heads = heads
+        slopes = torch.Tensor(self._get_slopes(heads))
+        slopes = rearrange(slopes, 'h -> () h () ()')
+        self.register_buffer('slopes', slopes, persistent=False)
+        self.register_buffer('bias', None, persistent=False)
+
+    @staticmethod
+    def _get_slopes(heads):
+        def get_slopes_power_of_2(n):
+            start = (2 ** (-2 ** -(math.log2(n) - 3)))
+            ratio = start
+            return [start * ratio ** i for i in range(n)]
+
+        if math.log2(heads).is_integer():
+            return get_slopes_power_of_2(heads)
+
+        closest_power_of_2 = 2 ** math.floor(math.log2(heads))
+        return get_slopes_power_of_2(closest_power_of_2) + get_slopes_power_of_2(2 * closest_power_of_2)[0::2][
+                                                           :heads - closest_power_of_2]
+
+    def forward(self, qk_dots):
+        h, i, j, device = *qk_dots.shape[-3:], qk_dots.device
+
+        if exists(self.bias) and self.bias.shape[-1] >= j:
+            return qk_dots + self.bias[..., :j]
+
+        bias = torch.arange(j, device=device)
+        bias = rearrange(bias, 'j -> () () () j')
+        bias = bias * self.slopes
+
+        num_heads_unalibied = h - bias.shape[1]
+        bias = F.pad(bias, (0, 0, 0, 0, 0, num_heads_unalibied))
+
+        self.register_buffer('bias', bias, persistent=False)
+        return qk_dots + self.bias
+
+
+class LearnedAlibiPositionalBias(AlibiPositionalBias):
+    def __init__(self, heads, bidirectional=False):
+        super().__init__(heads)
+        los_slopes = torch.log(self.slopes)
+        self.learned_logslopes = nn.Parameter(los_slopes)
+
+        self.bidirectional = bidirectional
+        if self.bidirectional:
+            self.learned_logslopes_future = nn.Parameter(los_slopes)
+
+    def forward(self, qk_dots):
+        h, i, j, device = *qk_dots.shape[-3:], qk_dots.device
+
+        def get_slopes(param):
+            return F.pad(param.exp(), (0, 0, 0, 0, 0, h - param.shape[1]))
+
+        if exists(self.bias) and self.bias.shape[-1] >= j:
+            bias = self.bias[..., :i, :j]
+        else:
+            i_arange = torch.arange(i, device=device)
+            j_arange = torch.arange(j, device=device)
+            bias = rearrange(j_arange, 'j -> 1 1 1 j') - rearrange(i_arange, 'i -> 1 1 i 1')
+            self.register_buffer('bias', bias, persistent=False)
+
+        if self.bidirectional:
+            past_slopes = get_slopes(self.learned_logslopes)
+            future_slopes = get_slopes(self.learned_logslopes_future)
+            bias = torch.tril(bias * past_slopes) + torch.triu(bias * future_slopes)
+        else:
+            slopes = get_slopes(self.learned_logslopes)
+            bias = bias * slopes
+
+        return qk_dots + bias
+
+
+class RotaryEmbedding(nn.Module):
+    def __init__(self, dim):
+        super().__init__()
+        inv_freq = 1. / (10000 ** (torch.arange(0, dim, 2).float() / dim))
+        self.register_buffer('inv_freq', inv_freq)
+
+    def forward(self, max_seq_len, device):
+        t = torch.arange(max_seq_len, device=device).type_as(self.inv_freq)
+        freqs = torch.einsum('i , j -> i j', t, self.inv_freq)
+        emb = torch.cat((freqs, freqs), dim=-1)
+        return rearrange(emb, 'n d -> () () n d')
+
+
+def rotate_half(x):
+    x = rearrange(x, '... (j d) -> ... j d', j=2)
+    x1, x2 = x.unbind(dim=-2)
+    return torch.cat((-x2, x1), dim=-1)
+
+
+def apply_rotary_pos_emb(t, freqs):
+    seq_len = t.shape[-2]
+    freqs = freqs[:, :, -seq_len:]
+    return (t * freqs.cos()) + (rotate_half(t) * freqs.sin())
+
+
+# norms
+
+class Scale(nn.Module):
+    def __init__(self, value, fn):
+        super().__init__()
+        self.value = value
+        self.fn = fn
+
+    def forward(self, x, **kwargs):
+        out = self.fn(x, **kwargs)
+        scale_fn = lambda t: t * self.value
+
+        if not isinstance(out, tuple):
+            return scale_fn(out)
+
+        return (scale_fn(out[0]), *out[1:])
+
+
+class Rezero(nn.Module):
+    def __init__(self, fn):
+        super().__init__()
+        self.fn = fn
+        self.g = nn.Parameter(torch.zeros(1))
+
+    def forward(self, x, **kwargs):
+        out = self.fn(x, **kwargs)
+        rezero_fn = lambda t: t * self.g
+
+        if not isinstance(out, tuple):
+            return rezero_fn(out)
+
+        return (rezero_fn(out[0]), *out[1:])
+
+
+class ScaleNorm(nn.Module):
+    def __init__(self, dim, eps=1e-5):
+        super().__init__()
+        self.scale = dim ** -0.5
+        self.eps = eps
+        self.g = nn.Parameter(torch.ones(1))
+
+    def forward(self, x):
+        norm = torch.norm(x, dim=-1, keepdim=True) * self.scale
+        return x / norm.clamp(min=self.eps) * self.g
+
+
+class RMSNorm(nn.Module):
+    def __init__(self, dim, eps=1e-8):
+        super().__init__()
+        self.scale = dim ** -0.5
+        self.eps = eps
+        self.g = nn.Parameter(torch.ones(dim))
+
+    def forward(self, x):
+        norm = torch.norm(x, dim=-1, keepdim=True) * self.scale
+        return x / norm.clamp(min=self.eps) * self.g
+
+
+class RMSScaleShiftNorm(nn.Module):
+    def __init__(self, dim, eps=1e-8):
+        super().__init__()
+        self.scale = dim ** -0.5
+        self.eps = eps
+        self.g = nn.Parameter(torch.ones(dim))
+        self.scale_shift_process = nn.Linear(dim * 2, dim * 2)
+
+    def forward(self, x, norm_scale_shift_inp):
+        norm = torch.norm(x, dim=-1, keepdim=True) * self.scale
+        norm = x / norm.clamp(min=self.eps) * self.g
+
+        ss_emb = self.scale_shift_process(norm_scale_shift_inp)
+        scale, shift = torch.chunk(ss_emb, 2, dim=1)
+        h = norm * (1 + scale.unsqueeze(1)) + shift.unsqueeze(1)
+        return h
+
+
+# residual and residual gates
+
+class Residual(nn.Module):
+    def __init__(self, dim, scale_residual=False):
+        super().__init__()
+        self.residual_scale = nn.Parameter(torch.ones(dim)) if scale_residual else None
+
+    def forward(self, x, residual):
+        if exists(self.residual_scale):
+            residual = residual * self.residual_scale
+
+        return x + residual
+
+
+class GRUGating(nn.Module):
+    def __init__(self, dim, scale_residual=False):
+        super().__init__()
+        self.gru = nn.GRUCell(dim, dim)
+        self.residual_scale = nn.Parameter(torch.ones(dim)) if scale_residual else None
+
+    def forward(self, x, residual):
+        if exists(self.residual_scale):
+            residual = residual * self.residual_scale
+
+        gated_output = self.gru(
+            rearrange(x, 'b n d -> (b n) d'),
+            rearrange(residual, 'b n d -> (b n) d')
+        )
+
+        return gated_output.reshape_as(x)
+
+
+# token shifting
+
+def shift(t, amount, mask=None):
+    if amount == 0:
+        return t
+
+    if exists(mask):
+        t = t.masked_fill(~mask[..., None], 0.)
+
+    return F.pad(t, (0, 0, amount, -amount), value=0.)
+
+
+class ShiftTokens(nn.Module):
+    def __init__(self, shifts, fn):
+        super().__init__()
+        self.fn = fn
+        self.shifts = tuple(shifts)
+
+    def forward(self, x, **kwargs):
+        mask = kwargs.get('mask', None)
+        shifts = self.shifts
+        segments = len(shifts)
+        feats_per_shift = x.shape[-1] // segments
+        splitted = x.split(feats_per_shift, dim=-1)
+        segments_to_shift, rest = splitted[:segments], splitted[segments:]
+        segments_to_shift = list(map(lambda args: shift(*args, mask=mask), zip(segments_to_shift, shifts)))
+        x = torch.cat((*segments_to_shift, *rest), dim=-1)
+        return self.fn(x, **kwargs)
+
+
+# feedforward
+
+class GLU(nn.Module):
+    def __init__(self, dim_in, dim_out, activation):
+        super().__init__()
+        self.act = activation
+        self.proj = nn.Linear(dim_in, dim_out * 2)
+
+    def forward(self, x):
+        x, gate = self.proj(x).chunk(2, dim=-1)
+        return x * self.act(gate)
+
+
+class FeedForward(nn.Module):
+    def __init__(
+            self,
+            dim,
+            dim_out=None,
+            mult=4,
+            glu=False,
+            relu_squared=False,
+            post_act_ln=False,
+            dropout=0.,
+            zero_init_output=False
+    ):
+        super().__init__()
+        inner_dim = int(dim * mult)
+        dim_out = default(dim_out, dim)
+        activation = ReluSquared() if relu_squared else nn.GELU()
+
+        project_in = nn.Sequential(
+            nn.Linear(dim, inner_dim),
+            activation
+        ) if not glu else GLU(dim, inner_dim, activation)
+
+        self.net = nn.Sequential(
+            project_in,
+            nn.LayerNorm(inner_dim) if post_act_ln else nn.Identity(),
+            nn.Dropout(dropout),
+            nn.Linear(inner_dim, dim_out)
+        )
+
+        # init last linear layer to 0
+        if zero_init_output:
+            init_zero_(self.net[-1])
+
+    def forward(self, x):
+        return self.net(x)
+
+
+# attention.
+
+class Attention(nn.Module):
+    def __init__(
+            self,
+            dim,
+            dim_head=DEFAULT_DIM_HEAD,
+            heads=8,
+            causal=False,
+            talking_heads=False,
+            head_scale=False,
+            collab_heads=False,
+            collab_compression=.3,
+            sparse_topk=None,
+            use_entmax15=False,
+            num_mem_kv=0,
+            dropout=0.,
+            on_attn=False,
+            gate_values=False,
+            zero_init_output=False,
+            max_attend_past=None,
+            qk_norm=False,
+            scale_init_value=None,
+            rel_pos_bias=False,
+            rel_pos_num_buckets=32,
+            rel_pos_max_distance=128,
+    ):
+        super().__init__()
+        self.scale = dim_head ** -0.5
+
+        self.heads = heads
+        self.causal = causal
+        self.max_attend_past = max_attend_past
+
+        qk_dim = v_dim = dim_head * heads
+
+        # collaborative heads
+        self.collab_heads = collab_heads
+        if self.collab_heads:
+            qk_dim = int(collab_compression * qk_dim)
+            self.collab_mixing = nn.Parameter(torch.randn(heads, qk_dim))
+
+        self.to_q = nn.Linear(dim, qk_dim, bias=False)
+        self.to_k = nn.Linear(dim, qk_dim, bias=False)
+        self.to_v = nn.Linear(dim, v_dim, bias=False)
+
+        self.dropout = nn.Dropout(dropout)
+
+        # add GLU gating for aggregated values, from alphafold2
+        self.to_v_gate = None
+        if gate_values:
+            self.to_v_gate = nn.Linear(dim, v_dim)
+            nn.init.constant_(self.to_v_gate.weight, 0)
+            nn.init.constant_(self.to_v_gate.bias, 1)
+
+        # cosine sim attention
+        self.qk_norm = qk_norm
+        if qk_norm:
+            scale_init_value = default(scale_init_value,
+                                       -3)  # if not provided, initialize as though it were sequence length of 1024
+            self.scale = nn.Parameter(torch.ones(1, heads, 1, 1) * scale_init_value)
+
+        # talking heads
+        self.talking_heads = talking_heads
+        if talking_heads:
+            self.pre_softmax_proj = nn.Parameter(torch.randn(heads, heads))
+            self.post_softmax_proj = nn.Parameter(torch.randn(heads, heads))
+
+        # head scaling
+        self.head_scale = head_scale
+        if head_scale:
+            self.head_scale_params = nn.Parameter(torch.ones(1, heads, 1, 1))
+
+        # explicit topk sparse attention
+        self.sparse_topk = sparse_topk
+
+        # entmax
+        self.attn_fn = entmax15 if use_entmax15 else F.softmax
+
+        # add memory key / values
+        self.num_mem_kv = num_mem_kv
+        if num_mem_kv > 0:
+            self.mem_k = nn.Parameter(torch.randn(heads, num_mem_kv, dim_head))
+            self.mem_v = nn.Parameter(torch.randn(heads, num_mem_kv, dim_head))
+
+        # attention on attention
+        self.attn_on_attn = on_attn
+        self.to_out = nn.Sequential(nn.Linear(v_dim, dim * 2), nn.GLU()) if on_attn else nn.Linear(v_dim, dim)
+
+        self.rel_pos_bias = rel_pos_bias
+        if rel_pos_bias:
+            assert rel_pos_num_buckets <= rel_pos_max_distance, 'number of relative position buckets must be less than the relative position max distance'
+            self.rel_pos = RelativePositionBias(scale=dim_head ** 0.5, causal=causal, heads=heads,
+                                                num_buckets=rel_pos_num_buckets, max_distance=rel_pos_max_distance)
+
+        # init output projection 0
+        if zero_init_output:
+            init_zero_(self.to_out)
+
+    def forward(
+            self,
+            x,
+            context=None,
+            mask=None,
+            context_mask=None,
+            attn_mask=None,
+            sinusoidal_emb=None,
+            rotary_pos_emb=None,
+            prev_attn=None,
+            mem=None
+    ):
+        b, n, _, h, talking_heads, collab_heads, head_scale, scale, device, has_context = *x.shape, self.heads, self.talking_heads, self.collab_heads, self.head_scale, self.scale, x.device, exists(
+            context)
+        kv_input = default(context, x)
+
+        q_input = x
+        k_input = kv_input
+        v_input = kv_input
+
+        if exists(mem):
+            k_input = torch.cat((mem, k_input), dim=-2)
+            v_input = torch.cat((mem, v_input), dim=-2)
+
+        if exists(sinusoidal_emb):
+            # in shortformer, the query would start at a position offset depending on the past cached memory
+            offset = k_input.shape[-2] - q_input.shape[-2]
+            q_input = q_input + sinusoidal_emb(q_input, offset=offset)
+            k_input = k_input + sinusoidal_emb(k_input)
+
+        q = self.to_q(q_input)
+        k = self.to_k(k_input)
+        v = self.to_v(v_input)
+
+        if not collab_heads:
+            q, k, v = map(lambda t: rearrange(t, 'b n (h d) -> b h n d', h=h), (q, k, v))
+        else:
+            q = einsum('b i d, h d -> b h i d', q, self.collab_mixing)
+            k = rearrange(k, 'b n d -> b () n d')
+            v = rearrange(v, 'b n (h d) -> b h n d', h=h)
+
+        if exists(rotary_pos_emb) and not has_context:
+            l = rotary_pos_emb.shape[-1]
+            (ql, qr), (kl, kr), (vl, vr) = map(lambda t: (t[..., :l], t[..., l:]), (q, k, v))
+            ql, kl, vl = map(lambda t: apply_rotary_pos_emb(t, rotary_pos_emb), (ql, kl, vl))
+            q, k, v = map(lambda t: torch.cat(t, dim=-1), ((ql, qr), (kl, kr), (vl, vr)))
+
+        input_mask = None
+        if any(map(exists, (mask, context_mask))):
+            q_mask = default(mask, lambda: torch.ones((b, n), device=device).bool())
+            k_mask = q_mask if not exists(context) else context_mask
+            k_mask = default(k_mask, lambda: torch.ones((b, k.shape[-2]), device=device).bool())
+            q_mask = rearrange(q_mask, 'b i -> b () i ()')
+            k_mask = rearrange(k_mask, 'b j -> b () () j')
+            input_mask = q_mask * k_mask
+
+        if self.num_mem_kv > 0:
+            mem_k, mem_v = map(lambda t: repeat(t, 'h n d -> b h n d', b=b), (self.mem_k, self.mem_v))
+            k = torch.cat((mem_k, k), dim=-2)
+            v = torch.cat((mem_v, v), dim=-2)
+            if exists(input_mask):
+                input_mask = F.pad(input_mask, (self.num_mem_kv, 0), value=True)
+
+        if collab_heads:
+            k = k.expand(-1, h, -1, -1)
+
+        if self.qk_norm:
+            q, k = map(l2norm, (q, k))
+            scale = 1 / (self.scale.exp().clamp(min=1e-2))
+
+        dots = einsum('b h i d, b h j d -> b h i j', q, k) * scale
+        mask_value = max_neg_value(dots)
+
+        if exists(prev_attn):
+            dots = dots + prev_attn
+
+        pre_softmax_attn = dots.clone()
+
+        if talking_heads:
+            dots = einsum('b h i j, h k -> b k i j', dots, self.pre_softmax_proj).contiguous()
+
+        if self.rel_pos_bias:
+            dots = self.rel_pos(dots)
+
+        if exists(input_mask):
+            dots.masked_fill_(~input_mask, mask_value)
+            del input_mask
+
+        if exists(attn_mask):
+            assert 2 <= attn_mask.ndim <= 4, 'attention mask must have greater than 2 dimensions but less than or equal to 4'
+            if attn_mask.ndim == 2:
+                attn_mask = rearrange(attn_mask, 'i j -> () () i j')
+            elif attn_mask.ndim == 3:
+                attn_mask = rearrange(attn_mask, 'h i j -> () h i j')
+            dots.masked_fill_(~attn_mask, mask_value)
+
+        if exists(self.max_attend_past):
+            i, j = dots.shape[-2:]
+            range_q = torch.arange(j - i, j, device=device)
+            range_k = torch.arange(j, device=device)
+            dist = rearrange(range_q, 'i -> () () i ()') - rearrange(range_k, 'j -> () () () j')
+            mask = dist > self.max_attend_past
+            dots.masked_fill_(mask, mask_value)
+            del mask
+
+        if self.causal:
+            i, j = dots.shape[-2:]
+            r = torch.arange(i, device=device)
+            mask = rearrange(r, 'i -> () () i ()') < rearrange(r, 'j -> () () () j')
+            mask = F.pad(mask, (j - i, 0), value=False)
+            dots.masked_fill_(mask, mask_value)
+            del mask
+
+        if exists(self.sparse_topk) and self.sparse_topk < dots.shape[-1]:
+            top, _ = dots.topk(self.sparse_topk, dim=-1)
+            vk = top[..., -1].unsqueeze(-1).expand_as(dots)
+            mask = dots < vk
+            dots.masked_fill_(mask, mask_value)
+            del mask
+
+        attn = self.attn_fn(dots, dim=-1)
+        post_softmax_attn = attn.clone()
+
+        attn = self.dropout(attn)
+
+        if talking_heads:
+            attn = einsum('b h i j, h k -> b k i j', attn, self.post_softmax_proj).contiguous()
+
+        out = einsum('b h i j, b h j d -> b h i d', attn, v)
+
+        if head_scale:
+            out = out * self.head_scale_params
+
+        out = rearrange(out, 'b h n d -> b n (h d)')
+
+        if exists(self.to_v_gate):
+            gates = self.to_v_gate(x)
+            out = out * gates.sigmoid()
+
+        intermediates = Intermediates(
+            pre_softmax_attn=pre_softmax_attn,
+            post_softmax_attn=post_softmax_attn
+        )
+
+        return self.to_out(out), intermediates
+
+
+class AttentionLayers(nn.Module):
+    def __init__(
+            self,
+            dim,
+            depth,
+            heads=8,
+            causal=False,
+            cross_attend=False,
+            only_cross=False,
+            use_scalenorm=False,
+            use_rms_scaleshift_norm=False,
+            use_rmsnorm=False,
+            use_rezero=False,
+            alibi_pos_bias=False,
+            alibi_num_heads=None,
+            alibi_learned=False,
+            position_infused_attn=False,
+            rotary_pos_emb=False,
+            rotary_emb_dim=None,
+            custom_layers=None,
+            sandwich_coef=None,
+            par_ratio=None,
+            residual_attn=False,
+            cross_residual_attn=False,
+            macaron=False,
+            pre_norm=True,
+            gate_residual=False,
+            scale_residual=False,
+            shift_tokens=0,
+            sandwich_norm=False,
+            use_qk_norm_attn=False,
+            qk_norm_attn_seq_len=None,
+            zero_init_branch_output=False,
+            **kwargs
+    ):
+        super().__init__()
+        ff_kwargs, kwargs = groupby_prefix_and_trim('ff_', kwargs)
+        attn_kwargs, _ = groupby_prefix_and_trim('attn_', kwargs)
+
+        dim_head = attn_kwargs.get('dim_head', DEFAULT_DIM_HEAD)
+
+        self.dim = dim
+        self.depth = depth
+        self.layers = nn.ModuleList([])
+
+        rel_pos_bias = 'rel_pos_bias' in attn_kwargs
+        self.has_pos_emb = position_infused_attn or rel_pos_bias or rotary_pos_emb
+        self.pia_pos_emb = FixedPositionalEmbedding(dim) if position_infused_attn else None
+
+        rotary_emb_dim = max(default(rotary_emb_dim, dim_head // 2), 32)
+        self.rotary_pos_emb = RotaryEmbedding(rotary_emb_dim) if rotary_pos_emb else None
+
+        assert not (
+                    alibi_pos_bias and rel_pos_bias), 'you can only choose Alibi positional bias or T5 relative positional bias, not both'
+
+        if alibi_pos_bias:
+            alibi_num_heads = default(alibi_num_heads, heads)
+            assert alibi_num_heads <= heads, 'number of ALiBi heads must be less than the total number of heads'
+            alibi_pos_klass = LearnedAlibiPositionalBias if alibi_learned or not causal else AlibiPositionalBias
+            self.rel_pos = alibi_pos_klass(heads=alibi_num_heads, bidirectional=not causal)
+        else:
+            self.rel_pos = None
+
+        assert not (not pre_norm and sandwich_norm), 'sandwich norm cannot be used when not using prenorm'
+        self.pre_norm = pre_norm
+        self.sandwich_norm = sandwich_norm
+
+        self.residual_attn = residual_attn
+        self.cross_residual_attn = cross_residual_attn
+        self.cross_attend = cross_attend
+
+        norm_class = ScaleNorm if use_scalenorm else nn.LayerNorm
+        norm_class = RMSNorm if use_rmsnorm else norm_class
+        norm_class = RMSScaleShiftNorm if use_rms_scaleshift_norm else norm_class
+        norm_fn = partial(norm_class, dim)
+
+        norm_fn = nn.Identity if use_rezero else norm_fn
+        branch_fn = Rezero if use_rezero else None
+
+        if cross_attend and not only_cross:
+            default_block = ('a', 'c', 'f')
+        elif cross_attend and only_cross:
+            default_block = ('c', 'f')
+        else:
+            default_block = ('a', 'f')
+
+        if macaron:
+            default_block = ('f',) + default_block
+
+        # qk normalization
+
+        if use_qk_norm_attn:
+            attn_scale_init_value = -math.log(math.log2(qk_norm_attn_seq_len ** 2 - qk_norm_attn_seq_len)) if exists(
+                qk_norm_attn_seq_len) else None
+            attn_kwargs = {**attn_kwargs, 'qk_norm': True, 'scale_init_value': attn_scale_init_value}
+
+        # zero init
+
+        if zero_init_branch_output:
+            attn_kwargs = {**attn_kwargs, 'zero_init_output': True}
+            ff_kwargs = {**ff_kwargs, 'zero_init_output': True}
+
+        # calculate layer block order
+
+        if exists(custom_layers):
+            layer_types = custom_layers
+        elif exists(par_ratio):
+            par_depth = depth * len(default_block)
+            assert 1 < par_ratio <= par_depth, 'par ratio out of range'
+            default_block = tuple(filter(not_equals('f'), default_block))
+            par_attn = par_depth // par_ratio
+            depth_cut = par_depth * 2 // 3  # 2 / 3 attention layer cutoff suggested by PAR paper
+            par_width = (depth_cut + depth_cut // par_attn) // par_attn
+            assert len(default_block) <= par_width, 'default block is too large for par_ratio'
+            par_block = default_block + ('f',) * (par_width - len(default_block))
+            par_head = par_block * par_attn
+            layer_types = par_head + ('f',) * (par_depth - len(par_head))
+        elif exists(sandwich_coef):
+            assert sandwich_coef > 0 and sandwich_coef <= depth, 'sandwich coefficient should be less than the depth'
+            layer_types = ('a',) * sandwich_coef + default_block * (depth - sandwich_coef) + ('f',) * sandwich_coef
+        else:
+            layer_types = default_block * depth
+
+        self.layer_types = layer_types
+        self.num_attn_layers = len(list(filter(equals('a'), layer_types)))
+
+        # calculate token shifting
+
+        shift_tokens = cast_tuple(shift_tokens, len(layer_types))
+
+        # iterate and construct layers
+
+        for ind, (layer_type, layer_shift_tokens) in enumerate(zip(self.layer_types, shift_tokens)):
+            is_last_layer = ind == (len(self.layer_types) - 1)
+
+            if layer_type == 'a':
+                layer = Attention(dim, heads=heads, causal=causal, **attn_kwargs)
+            elif layer_type == 'c':
+                layer = Attention(dim, heads=heads, **attn_kwargs)
+            elif layer_type == 'f':
+                layer = FeedForward(dim, **ff_kwargs)
+                layer = layer if not macaron else Scale(0.5, layer)
+            else:
+                raise Exception(f'invalid layer type {layer_type}')
+
+            if layer_shift_tokens > 0:
+                shift_range_upper = layer_shift_tokens + 1
+                shift_range_lower = -layer_shift_tokens if not causal else 0
+                layer = ShiftTokens(range(shift_range_lower, shift_range_upper), layer)
+
+            if exists(branch_fn):
+                layer = branch_fn(layer)
+
+            residual_fn = GRUGating if gate_residual else Residual
+            residual = residual_fn(dim, scale_residual=scale_residual)
+
+            layer_uses_qk_norm = use_qk_norm_attn and layer_type in ('a', 'c')
+
+            pre_branch_norm = norm_fn() if pre_norm and not layer_uses_qk_norm else None
+            post_branch_norm = norm_fn() if sandwich_norm or layer_uses_qk_norm else None
+            post_main_norm = norm_fn() if not pre_norm and not is_last_layer else None
+
+            norms = nn.ModuleList([
+                pre_branch_norm,
+                post_branch_norm,
+                post_main_norm
+            ])
+
+            self.layers.append(nn.ModuleList([
+                norms,
+                layer,
+                residual
+            ]))
+
+    def forward(
+            self,
+            x,
+            context=None,
+            full_context=None,  # for passing a list of hidden states from an encoder
+            mask=None,
+            context_mask=None,
+            attn_mask=None,
+            mems=None,
+            return_hiddens=False,
+            norm_scale_shift_inp=None,
+    ):
+
+        assert not (self.cross_attend ^ (exists(context) or exists(
+            full_context))), 'context must be passed in if cross_attend is set to True'
+        assert context is None or full_context is None, 'only one of full_context or context can be provided'
+
+        hiddens = []
+        intermediates = []
+        prev_attn = None
+        prev_cross_attn = None
+
+        mems = mems.copy() if exists(mems) else [None] * self.num_attn_layers
+        norm_args = {}
+        if exists(norm_scale_shift_inp):
+            norm_args['norm_scale_shift_inp'] = norm_scale_shift_inp
+
+        rotary_pos_emb = None
+        if exists(self.rotary_pos_emb):
+            max_rotary_emb_length = max(list(map(lambda m: (m.shape[1] if exists(m) else 0) + x.shape[1], mems)))
+            rotary_pos_emb = self.rotary_pos_emb(max_rotary_emb_length, x.device)
+
+        cross_attn_count = 0
+        for ind, (layer_type, (norm, block, residual_fn)) in enumerate(zip(self.layer_types, self.layers)):
+            if layer_type == 'a':
+                layer_mem = mems.pop(0) if mems else None
+
+            residual = x
+
+            pre_branch_norm, post_branch_norm, post_main_norm = norm
+
+            if exists(pre_branch_norm):
+                x = pre_branch_norm(x, **norm_args)
+
+            if layer_type == 'a':
+                out, inter = checkpoint(block, x, None, mask, None, attn_mask, self.pia_pos_emb, rotary_pos_emb,
+                                        prev_attn, layer_mem)
+            elif layer_type == 'c':
+                if exists(full_context):
+                    out, inter = checkpoint(block, x, full_context[cross_attn_count], mask, context_mask, None, None,
+                                            None, prev_attn)
+                else:
+                    out, inter = checkpoint(block, x, context, mask, context_mask, None, None, None, prev_attn)
+            elif layer_type == 'f':
+                out = checkpoint(block, x)
+
+            if exists(post_branch_norm):
+                out = post_branch_norm(out, **norm_args)
+
+            x = residual_fn(out, residual)
+
+            if layer_type in ('a', 'c'):
+                intermediates.append(inter)
+
+            if layer_type == 'a' and self.residual_attn:
+                prev_attn = inter.pre_softmax_attn
+            elif layer_type == 'c' and self.cross_residual_attn:
+                prev_cross_attn = inter.pre_softmax_attn
+
+            if exists(post_main_norm):
+                x = post_main_norm(x, **norm_args)
+
+            if layer_type == 'c':
+                cross_attn_count += 1
+
+            if layer_type == 'f':
+                hiddens.append(x)
+
+        if return_hiddens:
+            intermediates = LayerIntermediates(
+                hiddens=hiddens,
+                attn_intermediates=intermediates
+            )
+
+            return x, intermediates
+
+        return x
+
+
+class Encoder(AttentionLayers):
+    def __init__(self, **kwargs):
+        assert 'causal' not in kwargs, 'cannot set causality on encoder'
+        super().__init__(causal=False, **kwargs)
+
+
+class Decoder(AttentionLayers):
+    def __init__(self, **kwargs):
+        assert 'causal' not in kwargs, 'cannot set causality on decoder'
+        super().__init__(causal=True, **kwargs)
+
+
+class CrossAttender(AttentionLayers):
+    def __init__(self, **kwargs):
+        super().__init__(cross_attend=True, only_cross=True, **kwargs)
+
+
+class ViTransformerWrapper(nn.Module):
+    def __init__(
+            self,
+            *,
+            image_size,
+            patch_size,
+            attn_layers,
+            num_classes=None,
+            dropout=0.,
+            emb_dropout=0.
+    ):
+        super().__init__()
+        assert isinstance(attn_layers, Encoder), 'attention layers must be an Encoder'
+        assert image_size % patch_size == 0, 'image dimensions must be divisible by the patch size'
+        dim = attn_layers.dim
+        num_patches = (image_size // patch_size) ** 2
+        patch_dim = 3 * patch_size ** 2
+
+        self.patch_size = patch_size
+
+        self.pos_embedding = nn.Parameter(torch.randn(1, num_patches + 1, dim))
+        self.patch_to_embedding = nn.Linear(patch_dim, dim)
+        self.cls_token = nn.Parameter(torch.randn(1, 1, dim))
+        self.dropout = nn.Dropout(emb_dropout)
+
+        self.attn_layers = attn_layers
+        self.norm = nn.LayerNorm(dim)
+        self.mlp_head = FeedForward(dim, dim_out=num_classes, dropout=dropout) if exists(num_classes) else None
+
+    def forward(
+            self,
+            img,
+            return_embeddings=False
+    ):
+        p = self.patch_size
+
+        x = rearrange(img, 'b c (h p1) (w p2) -> b (h w) (p1 p2 c)', p1=p, p2=p)
+        x = self.patch_to_embedding(x)
+        b, n, _ = x.shape
+
+        cls_tokens = repeat(self.cls_token, '() n d -> b n d', b=b)
+        x = torch.cat((cls_tokens, x), dim=1)
+        x = x + self.pos_embedding[:, :(n + 1)]
+        x = self.dropout(x)
+
+        x = self.attn_layers(x)
+        x = self.norm(x)
+
+        if not exists(self.mlp_head) or return_embeddings:
+            return x
+
+        return self.mlp_head(x[:, 0])
+
+
+class TransformerWrapper(nn.Module):
+    def __init__(
+            self,
+            *,
+            num_tokens,
+            max_seq_len,
+            attn_layers,
+            emb_dim=None,
+            max_mem_len=0.,
+            shift_mem_down=0,
+            emb_dropout=0.,
+            num_memory_tokens=None,
+            tie_embedding=False,
+            use_pos_emb=True
+    ):
+        super().__init__()
+        assert isinstance(attn_layers, AttentionLayers), 'attention layers must be one of Encoder or Decoder'
+
+        dim = attn_layers.dim
+        emb_dim = default(emb_dim, dim)
+
+        self.max_seq_len = max_seq_len
+        self.max_mem_len = max_mem_len
+        self.shift_mem_down = shift_mem_down
+
+        self.token_emb = nn.Embedding(num_tokens, emb_dim)
+        self.pos_emb = AbsolutePositionalEmbedding(emb_dim, max_seq_len) if (
+                    use_pos_emb and not attn_layers.has_pos_emb) else always(0)
+        self.emb_dropout = nn.Dropout(emb_dropout)
+
+        self.project_emb = nn.Linear(emb_dim, dim) if emb_dim != dim else nn.Identity()
+        self.attn_layers = attn_layers
+        self.norm = nn.LayerNorm(dim)
+
+        self.init_()
+
+        self.to_logits = nn.Linear(dim, num_tokens) if not tie_embedding else lambda t: t @ self.token_emb.weight.t()
+
+        # memory tokens (like [cls]) from Memory Transformers paper
+        num_memory_tokens = default(num_memory_tokens, 0)
+        self.num_memory_tokens = num_memory_tokens
+        if num_memory_tokens > 0:
+            self.memory_tokens = nn.Parameter(torch.randn(num_memory_tokens, dim))
+
+    def init_(self):
+        nn.init.kaiming_normal_(self.token_emb.weight)
+
+    def forward(
+            self,
+            x,
+            return_embeddings=False,
+            mask=None,
+            return_hiddens=False,
+            return_attn=False,
+            mems=None,
+            **kwargs
+    ):
+        b, n, device, num_mem = *x.shape, x.device, self.num_memory_tokens
+        x = self.token_emb(x)
+        x = x + self.pos_emb(x)
+        x = self.emb_dropout(x)
+
+        x = self.project_emb(x)
+
+        if num_mem > 0:
+            mem = repeat(self.memory_tokens, 'n d -> b n d', b=b)
+            x = torch.cat((mem, x), dim=1)
+
+            # auto-handle masking after appending memory tokens
+            if exists(mask):
+                mask = F.pad(mask, (num_mem, 0), value=True)
+
+        if self.shift_mem_down and exists(mems):
+            mems_l, mems_r = mems[:self.shift_mem_down], mems[self.shift_mem_down:]
+            mems = [*mems_r, *mems_l]
+
+        x, intermediates = self.attn_layers(x, mask=mask, mems=mems, return_hiddens=True, **kwargs)
+        x = self.norm(x)
+
+        mem, x = x[:, :num_mem], x[:, num_mem:]
+
+        out = self.to_logits(x) if not return_embeddings else x
+
+        if return_hiddens:
+            hiddens = intermediates.hiddens
+            return out, hiddens
+
+        if return_attn:
+            attn_maps = list(map(lambda t: t.post_softmax_attn, intermediates.attn_intermediates))
+            return out, attn_maps
+
+        return out
+
+
+class ContinuousTransformerWrapper(nn.Module):
+    def __init__(
+            self,
+            *,
+            max_seq_len,
+            attn_layers,
+            dim_in=None,
+            dim_out=None,
+            emb_dim=None,
+            emb_dropout=0.,
+            use_pos_emb=True
+    ):
+        super().__init__()
+        assert isinstance(attn_layers, AttentionLayers), 'attention layers must be one of Encoder or Decoder'
+
+        dim = attn_layers.dim
+
+        self.max_seq_len = max_seq_len
+
+        self.pos_emb = AbsolutePositionalEmbedding(dim, max_seq_len) if (
+                    use_pos_emb and not attn_layers.has_pos_emb) else always(0)
+        self.emb_dropout = nn.Dropout(emb_dropout)
+
+        self.project_in = nn.Linear(dim_in, dim) if exists(dim_in) else nn.Identity()
+
+        self.attn_layers = attn_layers
+        self.norm = nn.LayerNorm(dim)
+
+        self.project_out = nn.Linear(dim, dim_out) if exists(dim_out) else nn.Identity()
+
+    def forward(
+            self,
+            x,
+            return_embeddings=False,
+            mask=None,
+            return_attn=False,
+            mems=None,
+            **kwargs
+    ):
+        b, n, _, device = *x.shape, x.device
+
+        x = self.project_in(x)
+        x = x + self.pos_emb(x)
+        x = self.emb_dropout(x)
+
+        x, intermediates = self.attn_layers(x, mask=mask, mems=mems, return_hiddens=True, **kwargs)
+        x = self.norm(x)
+
+        out = self.project_out(x) if not return_embeddings else x
+
+        if return_attn:
+            attn_maps = list(map(lambda t: t.post_softmax_attn, intermediates.attn_intermediates))
+            return out, attn_maps
+
+        return out
+
+
+class XTransformer(nn.Module):
+    def __init__(
+            self,
+            *,
+            dim,
+            tie_token_emb=False,
+            **kwargs
+    ):
+        super().__init__()
+        enc_kwargs, kwargs = groupby_prefix_and_trim('enc_', kwargs)
+        dec_kwargs, kwargs = groupby_prefix_and_trim('dec_', kwargs)
+
+        assert 'dim' not in enc_kwargs and 'dim' not in dec_kwargs, 'dimension of either encoder or decoder must be set with `dim` keyword'
+        enc_transformer_kwargs = pick_and_pop(['num_tokens', 'max_seq_len'], enc_kwargs)
+        enc_transformer_kwargs['emb_dropout'] = enc_kwargs.pop('emb_dropout', 0)
+        enc_transformer_kwargs['num_memory_tokens'] = enc_kwargs.pop('num_memory_tokens', None)
+        enc_transformer_kwargs['use_pos_emb'] = enc_kwargs.pop('use_pos_emb', True)
+
+        dec_transformer_kwargs = pick_and_pop(['num_tokens', 'max_seq_len'], dec_kwargs)
+        dec_transformer_kwargs['emb_dropout'] = dec_kwargs.pop('emb_dropout', 0)
+        dec_transformer_kwargs['use_pos_emb'] = dec_kwargs.pop('use_pos_emb', True)
+
+        self.encoder = TransformerWrapper(
+            **enc_transformer_kwargs,
+            attn_layers=Encoder(dim=dim, **enc_kwargs)
+        )
+
+        self.decoder = TransformerWrapper(
+            **dec_transformer_kwargs,
+            attn_layers=Decoder(dim=dim, cross_attend=True, **dec_kwargs)
+        )
+
+        if tie_token_emb:
+            self.decoder.token_emb = self.encoder.token_emb
+
+        self.decoder = AutoregressiveWrapper(self.decoder)
+
+    @torch.no_grad()
+    def generate(self, seq_in, seq_out_start, seq_len, src_mask=None, src_attn_mask=None, **kwargs):
+        encodings = self.encoder(seq_in, mask=src_mask, attn_mask=src_attn_mask, return_embeddings=True)
+        return self.decoder.generate(seq_out_start, seq_len, context=encodings, context_mask=src_mask, **kwargs)
+
+    def forward(self, src, tgt, src_mask=None, tgt_mask=None, src_attn_mask=None):
+        enc = self.encoder(src, mask=src_mask, attn_mask=src_attn_mask, return_embeddings=True)
+        out = self.decoder(tgt, context=enc, mask=tgt_mask, context_mask=src_mask)
+        return out