from typing import Optional, Tuple import torch from torch import Tensor from torch.nn import Linear, Module from torch.nn import functional as F from torch.nn.init import constant_, xavier_normal_, xavier_uniform_ from torch.nn.modules.linear import NonDynamicallyQuantizableLinear from torch.nn.parameter import Parameter class MultiheadAttention(Module): r"""Allows the model to jointly attend to information from different representation subspaces as described in the paper: `Attention Is All You Need `_. Multi-Head Attention is defined as: .. math:: \text{MultiHead}(Q, K, V) = \text{Concat}(head_1,\dots,head_h)W^O where :math:`head_i = \text{Attention}(QW_i^Q, KW_i^K, VW_i^V)`. ``forward()`` will use a special optimized implementation if all of the following conditions are met: - self attention is being computed (i.e., ``query``, ``key``, and ``value`` are the same tensor. This restriction will be loosened in the future.) - Either autograd is disabled (using ``torch.inference_mode`` or ``torch.no_grad``) or no tensor argument ``requires_grad`` - training is disabled (using ``.eval()``) - dropout is 0 - ``add_bias_kv`` is ``False`` - ``add_zero_attn`` is ``False`` - ``batch_first`` is ``True`` and the input is batched - ``kdim`` and ``vdim`` are equal to ``embed_dim`` - at most one of ``key_padding_mask`` or ``attn_mask`` is passed - if a `NestedTensor `_ is passed, neither ``key_padding_mask`` nor ``attn_mask`` is passed If the optimized implementation is in use, a `NestedTensor `_ can be passed for ``query``/``key``/``value`` to represent padding more efficiently than using a padding mask. In this case, a `NestedTensor `_ will be returned, and an additional speedup proportional to the fraction of the input that is padding can be expected. Args: embed_dim: Total dimension of the model. num_heads: Number of parallel attention heads. Note that ``embed_dim`` will be split across ``num_heads`` (i.e. each head will have dimension ``embed_dim // num_heads``). dropout: Dropout probability on ``attn_output_weights``. Default: ``0.0`` (no dropout). bias: If specified, adds bias to input / output projection layers. Default: ``True``. add_bias_kv: If specified, adds bias to the key and value sequences at dim=0. Default: ``False``. add_zero_attn: If specified, adds a new batch of zeros to the key and value sequences at dim=1. Default: ``False``. kdim: Total number of features for keys. Default: ``None`` (uses ``kdim=embed_dim``). vdim: Total number of features for values. Default: ``None`` (uses ``vdim=embed_dim``). batch_first: If ``True``, then the input and output tensors are provided as (batch, seq, feature). Default: ``False`` (seq, batch, feature). Examples:: >>> # xdoctest: +SKIP >>> multihead_attn = nn.MultiheadAttention(embed_dim, num_heads) >>> attn_output, attn_output_weights = multihead_attn(query, key, value) """ __constants__ = ["batch_first"] bias_k: Optional[torch.Tensor] bias_v: Optional[torch.Tensor] def __init__( self, embed_dim, num_heads, dropout=0.0, bias=True, add_bias_kv=False, add_zero_attn=False, kdim=None, vdim=None, batch_first=False, linear1_cls=Linear, linear2_cls=Linear, device=None, dtype=None, ) -> None: factory_kwargs = {"device": device, "dtype": dtype} super(MultiheadAttention, self).__init__() self.embed_dim = embed_dim self.kdim = kdim if kdim is not None else embed_dim self.vdim = vdim if vdim is not None else embed_dim self._qkv_same_embed_dim = self.kdim == embed_dim and self.vdim == embed_dim self.num_heads = num_heads self.dropout = dropout self.batch_first = batch_first self.head_dim = embed_dim // num_heads assert ( self.head_dim * num_heads == self.embed_dim ), "embed_dim must be divisible by num_heads" if add_bias_kv: self.bias_k = Parameter(torch.empty((1, 1, embed_dim), **factory_kwargs)) self.bias_v = Parameter(torch.empty((1, 1, embed_dim), **factory_kwargs)) else: self.bias_k = self.bias_v = None if linear1_cls == Linear: if not self._qkv_same_embed_dim: self.q_proj_weight = Parameter( torch.empty((embed_dim, embed_dim), **factory_kwargs) ) self.k_proj_weight = Parameter( torch.empty((embed_dim, self.kdim), **factory_kwargs) ) self.v_proj_weight = Parameter( torch.empty((embed_dim, self.vdim), **factory_kwargs) ) self.register_parameter("in_proj_weight", None) else: self.in_proj_weight = Parameter( torch.empty((3 * embed_dim, embed_dim), **factory_kwargs) ) self.register_parameter("q_proj_weight", None) self.register_parameter("k_proj_weight", None) self.register_parameter("v_proj_weight", None) if bias: self.in_proj_bias = Parameter( torch.empty(3 * embed_dim, **factory_kwargs) ) else: self.register_parameter("in_proj_bias", None) self.out_proj = NonDynamicallyQuantizableLinear( embed_dim, embed_dim, bias=bias, **factory_kwargs ) self._reset_parameters() else: if not self._qkv_same_embed_dim: raise NotImplementedError else: self.in_proj_linear = linear1_cls( embed_dim, 3 * embed_dim, bias=bias, **factory_kwargs ) self.in_proj_weight = self.in_proj_linear.weight self.register_parameter("q_proj_weight", None) self.register_parameter("k_proj_weight", None) self.register_parameter("v_proj_weight", None) if bias: self.in_proj_bias = self.in_proj_linear.bias else: self.register_parameter("in_proj_bias", None) self.out_proj = linear2_cls( embed_dim, embed_dim, bias=bias, **factory_kwargs ) if self.bias_k is not None: xavier_normal_(self.bias_k) if self.bias_v is not None: xavier_normal_(self.bias_v) self.add_zero_attn = add_zero_attn def _reset_parameters(self): if self._qkv_same_embed_dim: xavier_uniform_(self.in_proj_weight) else: xavier_uniform_(self.q_proj_weight) xavier_uniform_(self.k_proj_weight) xavier_uniform_(self.v_proj_weight) if self.in_proj_bias is not None: constant_(self.in_proj_bias, 0.0) constant_(self.out_proj.bias, 0.0) if self.bias_k is not None: xavier_normal_(self.bias_k) if self.bias_v is not None: xavier_normal_(self.bias_v) def __setstate__(self, state): if "_qkv_same_embed_dim" not in state: state["_qkv_same_embed_dim"] = True super(MultiheadAttention, self).__setstate__(state) def forward( self, query: Tensor, key: Tensor, value: Tensor, key_padding_mask: Optional[Tensor] = None, need_weights: bool = True, attn_mask: Optional[Tensor] = None, average_attn_weights: bool = True, ) -> Tuple[Tensor, Optional[Tensor]]: r""" Args: query: Query embeddings of shape :math:`(L, E_q)` for unbatched input, :math:`(L, N, E_q)` when ``batch_first=False`` or :math:`(N, L, E_q)` when ``batch_first=True``, where :math:`L` is the target sequence length, :math:`N` is the batch size, and :math:`E_q` is the query embedding dimension ``embed_dim``. Queries are compared against key-value pairs to produce the output. See "Attention Is All You Need" for more details. key: Key embeddings of shape :math:`(S, E_k)` for unbatched input, :math:`(S, N, E_k)` when ``batch_first=False`` or :math:`(N, S, E_k)` when ``batch_first=True``, where :math:`S` is the source sequence length, :math:`N` is the batch size, and :math:`E_k` is the key embedding dimension ``kdim``. See "Attention Is All You Need" for more details. value: Value embeddings of shape :math:`(S, E_v)` for unbatched input, :math:`(S, N, E_v)` when ``batch_first=False`` or :math:`(N, S, E_v)` when ``batch_first=True``, where :math:`S` is the source sequence length, :math:`N` is the batch size, and :math:`E_v` is the value embedding dimension ``vdim``. See "Attention Is All You Need" for more details. key_padding_mask: If specified, a mask of shape :math:`(N, S)` indicating which elements within ``key`` to ignore for the purpose of attention (i.e. treat as "padding"). For unbatched `query`, shape should be :math:`(S)`. Binary and byte masks are supported. For a binary mask, a ``True`` value indicates that the corresponding ``key`` value will be ignored for the purpose of attention. For a float mask, it will be directly added to the corresponding ``key`` value. need_weights: If specified, returns ``attn_output_weights`` in addition to ``attn_outputs``. Default: ``True``. attn_mask: If specified, a 2D or 3D mask preventing attention to certain positions. Must be of shape :math:`(L, S)` or :math:`(N\cdot\text{num\_heads}, L, S)`, where :math:`N` is the batch size, :math:`L` is the target sequence length, and :math:`S` is the source sequence length. A 2D mask will be broadcasted across the batch while a 3D mask allows for a different mask for each entry in the batch. Binary, byte, and float masks are supported. For a binary mask, a ``True`` value indicates that the corresponding position is not allowed to attend. For a byte mask, a non-zero value indicates that the corresponding position is not allowed to attend. For a float mask, the mask values will be added to the attention weight. average_attn_weights: If true, indicates that the returned ``attn_weights`` should be averaged across heads. Otherwise, ``attn_weights`` are provided separately per head. Note that this flag only has an effect when ``need_weights=True``. Default: ``True`` (i.e. average weights across heads) Outputs: - **attn_output** - Attention outputs of shape :math:`(L, E)` when input is unbatched, :math:`(L, N, E)` when ``batch_first=False`` or :math:`(N, L, E)` when ``batch_first=True``, where :math:`L` is the target sequence length, :math:`N` is the batch size, and :math:`E` is the embedding dimension ``embed_dim``. - **attn_output_weights** - Only returned when ``need_weights=True``. If ``average_attn_weights=True``, returns attention weights averaged across heads of shape :math:`(L, S)` when input is unbatched or :math:`(N, L, S)`, where :math:`N` is the batch size, :math:`L` is the target sequence length, and :math:`S` is the source sequence length. If ``average_attn_weights=False``, returns attention weights per head of shape :math:`(\text{num\_heads}, L, S)` when input is unbatched or :math:`(N, \text{num\_heads}, L, S)`. .. note:: `batch_first` argument is ignored for unbatched inputs. """ is_batched = query.dim() == 3 if key_padding_mask is not None: _kpm_dtype = key_padding_mask.dtype if _kpm_dtype != torch.bool and not torch.is_floating_point( key_padding_mask ): raise AssertionError( "only bool and floating types of key_padding_mask are supported" ) why_not_fast_path = "" if not is_batched: why_not_fast_path = ( f"input not batched; expected query.dim() of 3 but got {query.dim()}" ) elif query is not key or key is not value: # When lifting this restriction, don't forget to either # enforce that the dtypes all match or test cases where # they don't! why_not_fast_path = "non-self attention was used (query, key, and value are not the same Tensor)" elif self.in_proj_bias is not None and query.dtype != self.in_proj_bias.dtype: why_not_fast_path = f"dtypes of query ({query.dtype}) and self.in_proj_bias ({self.in_proj_bias.dtype}) don't match" elif ( self.in_proj_weight is not None and query.dtype != self.in_proj_weight.dtype ): # this case will fail anyway, but at least they'll get a useful error message. why_not_fast_path = f"dtypes of query ({query.dtype}) and self.in_proj_weight ({self.in_proj_weight.dtype}) don't match" elif self.training: why_not_fast_path = "training is enabled" elif not self.batch_first: why_not_fast_path = "batch_first was not True" elif self.bias_k is not None: why_not_fast_path = "self.bias_k was not None" elif self.bias_v is not None: why_not_fast_path = "self.bias_v was not None" elif self.dropout: why_not_fast_path = f"dropout was {self.dropout}, required zero" elif self.add_zero_attn: why_not_fast_path = "add_zero_attn was enabled" elif not self._qkv_same_embed_dim: why_not_fast_path = "_qkv_same_embed_dim was not True" elif attn_mask is not None: why_not_fast_path = "attn_mask was not None" elif query.is_nested and key_padding_mask is not None: why_not_fast_path = ( "key_padding_mask is not supported with NestedTensor input" ) elif self.num_heads % 2 == 1: why_not_fast_path = "num_heads is odd" elif torch.is_autocast_enabled(): why_not_fast_path = "autocast is enabled" if not why_not_fast_path: tensor_args = ( query, key, value, self.in_proj_weight, self.in_proj_bias, self.out_proj.weight, self.out_proj.bias, ) # We have to use list comprehensions below because TorchScript does not support # generator expressions. if torch.overrides.has_torch_function(tensor_args): why_not_fast_path = "some Tensor argument has_torch_function" elif not all( [ (x is None or x.is_cuda or "cpu" in str(x.device)) for x in tensor_args ] ): why_not_fast_path = "some Tensor argument is neither CUDA nor CPU" elif torch.is_grad_enabled() and any( [x is not None and x.requires_grad for x in tensor_args] ): why_not_fast_path = ( "grad is enabled and at least one of query or the " "input/output projection weights or biases requires_grad" ) if not why_not_fast_path: return torch._native_multi_head_attention( query, key, value, self.embed_dim, self.num_heads, self.in_proj_weight, self.in_proj_bias, self.out_proj.weight, self.out_proj.bias, key_padding_mask if key_padding_mask is not None else attn_mask, need_weights, average_attn_weights, 1 if key_padding_mask is not None else 0 if attn_mask is not None else None, ) any_nested = query.is_nested or key.is_nested or value.is_nested assert not any_nested, ( "MultiheadAttention does not support NestedTensor outside of its fast path. " + f"The fast path was not hit because {why_not_fast_path}" ) if self.batch_first and is_batched: # make sure that the transpose op does not affect the "is" property if key is value: if query is key: query = key = value = query.transpose(1, 0) else: query, key = [x.transpose(1, 0) for x in (query, key)] value = key else: query, key, value = [x.transpose(1, 0) for x in (query, key, value)] if not self._qkv_same_embed_dim: attn_output, attn_output_weights = F.multi_head_attention_forward( query, key, value, self.embed_dim, self.num_heads, self.in_proj_weight, self.in_proj_bias, self.bias_k, self.bias_v, self.add_zero_attn, self.dropout, self.out_proj.weight, self.out_proj.bias, training=self.training, key_padding_mask=key_padding_mask, need_weights=need_weights, attn_mask=attn_mask, use_separate_proj_weight=True, q_proj_weight=self.q_proj_weight, k_proj_weight=self.k_proj_weight, v_proj_weight=self.v_proj_weight, average_attn_weights=average_attn_weights, ) else: attn_output, attn_output_weights = F.multi_head_attention_forward( query, key, value, self.embed_dim, self.num_heads, self.in_proj_weight, self.in_proj_bias, self.bias_k, self.bias_v, self.add_zero_attn, self.dropout, self.out_proj.weight, self.out_proj.bias, training=self.training, key_padding_mask=key_padding_mask, need_weights=need_weights, attn_mask=attn_mask, average_attn_weights=average_attn_weights, ) if self.batch_first and is_batched: return attn_output.transpose(1, 0), attn_output_weights else: return attn_output, attn_output_weights