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# Copyright (c) Meta Platforms, Inc. and affiliates.
#
# This source code is licensed under the Apache License, Version 2.0
# found in the LICENSE file in the root directory of this source tree.
# References:
# https://github.com/facebookresearch/dino/blob/master/vision_transformer.py
# https://github.com/rwightman/pytorch-image-models/tree/master/timm/models/vision_transformer.py
import torch.nn.functional as F
from torch import nn, Tensor
XFORMERS_AVAILABLE = False
class Attention(nn.Module):
def __init__(
self,
dim: int,
num_heads: int = 8,
qkv_bias: bool = True,
proj_bias: bool = True,
attn_drop: float = 0.0,
proj_drop: float = 0.0,
norm_layer: nn.Module = nn.LayerNorm,
qk_norm: bool = False,
fused_attn: bool = True, # use F.scaled_dot_product_attention or not
rope=None,
) -> None:
super().__init__()
assert dim % num_heads == 0, "dim should be divisible by num_heads"
self.num_heads = num_heads
self.head_dim = dim // num_heads
self.scale = self.head_dim**-0.5
self.fused_attn = fused_attn
self.qkv = nn.Linear(dim, dim * 3, bias=qkv_bias)
self.q_norm = norm_layer(self.head_dim) if qk_norm else nn.Identity()
self.k_norm = norm_layer(self.head_dim) if qk_norm else nn.Identity()
self.attn_drop = nn.Dropout(attn_drop)
self.proj = nn.Linear(dim, dim, bias=proj_bias)
self.proj_drop = nn.Dropout(proj_drop)
self.rope = rope
def forward(self, x: Tensor, pos=None) -> Tensor:
B, N, C = x.shape
qkv = (
self.qkv(x)
.reshape(B, N, 3, self.num_heads, self.head_dim)
.permute(2, 0, 3, 1, 4)
)
q, k, v = qkv.unbind(0)
q, k = self.q_norm(q), self.k_norm(k)
if self.rope is not None:
q = self.rope(q, pos)
k = self.rope(k, pos)
if self.fused_attn:
x = F.scaled_dot_product_attention(
q,
k,
v,
dropout_p=self.attn_drop.p if self.training else 0.0,
)
else:
q = q * self.scale
attn = q @ k.transpose(-2, -1)
attn = attn.softmax(dim=-1)
attn = self.attn_drop(attn)
x = attn @ v
x = x.transpose(1, 2).reshape(B, N, C)
x = self.proj(x)
x = self.proj_drop(x)
return x
# class MemEffAttention(Attention):
# def forward(self, x: Tensor, attn_bias=None, pos=None) -> Tensor:
# assert pos is None
# if not XFORMERS_AVAILABLE:
# if attn_bias is not None:
# raise AssertionError("xFormers is required for using nested tensors")
# return super().forward(x)
# B, N, C = x.shape
# qkv = self.qkv(x).reshape(B, N, 3, self.num_heads, C // self.num_heads)
# q, k, v = unbind(qkv, 2)
# x = memory_efficient_attention(q, k, v, attn_bias=attn_bias)
# x = x.reshape([B, N, C])
# x = self.proj(x)
# x = self.proj_drop(x)
# return x
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