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import torch
import torch.nn as nn
from einops import rearrange
try: # v1
from flash_attn.flash_attn_interface import \
flash_attn_unpadded_qkvpacked_func
except: # v2
from flash_attn.flash_attn_interface import flash_attn_varlen_qkvpacked_func as flash_attn_unpadded_qkvpacked_func
from flash_attn.bert_padding import pad_input, unpad_input
class FlashAttention(nn.Module):
"""Implement the scaled dot product attention with softmax.
Arguments
---------
softmax_scale: The temperature to use for the softmax attention.
(default: 1/sqrt(d_keys) where d_keys is computed at
runtime)
attention_dropout: The dropout rate to apply to the attention
(default: 0.0)
"""
def __init__(self, softmax_scale=None, attention_dropout=0.0, device=None, dtype=None):
super().__init__()
self.softmax_scale = softmax_scale
self.dropout_p = attention_dropout
def forward(self, qkv, key_padding_mask=None, causal=False, cu_seqlens=None,
max_s=None, need_weights=False):
"""Implements the multihead softmax attention.
Arguments
---------
qkv: The tensor containing the query, key, and value. (B, S, 3, H, D) if key_padding_mask is None
if unpadded: (nnz, 3, h, d)
key_padding_mask: a bool tensor of shape (B, S)
"""
assert not need_weights
assert qkv.dtype in [torch.float16, torch.bfloat16]
assert qkv.is_cuda
if cu_seqlens is None:
batch_size = qkv.shape[0]
seqlen = qkv.shape[1]
if key_padding_mask is None:
qkv = rearrange(qkv, 'b s ... -> (b s) ...')
max_s = seqlen
cu_seqlens = torch.arange(0, (batch_size + 1) * seqlen, step=seqlen, dtype=torch.int32,
device=qkv.device)
output = flash_attn_unpadded_qkvpacked_func(
qkv, cu_seqlens, max_s, self.dropout_p if self.training else 0.0,
softmax_scale=self.softmax_scale, causal=causal
)
output = rearrange(output, '(b s) ... -> b s ...', b=batch_size)
else:
nheads = qkv.shape[-2]
x = rearrange(qkv, 'b s three h d -> b s (three h d)')
x_unpad, indices, cu_seqlens, max_s = unpad_input(x, key_padding_mask)
x_unpad = rearrange(x_unpad, 'nnz (three h d) -> nnz three h d', three=3, h=nheads)
output_unpad = flash_attn_unpadded_qkvpacked_func(
x_unpad, cu_seqlens, max_s, self.dropout_p if self.training else 0.0,
softmax_scale=self.softmax_scale, causal=causal
)
output = rearrange(pad_input(rearrange(output_unpad, 'nnz h d -> nnz (h d)'),
indices, batch_size, seqlen),
'b s (h d) -> b s h d', h=nheads)
else:
assert max_s is not None
output = flash_attn_unpadded_qkvpacked_func(
qkv, cu_seqlens, max_s, self.dropout_p if self.training else 0.0,
softmax_scale=self.softmax_scale, causal=causal
)
return output, None
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