# Copyright (c) Facebook, Inc. and its affiliates.
#
# This source code is licensed under the MIT license found in the
# LICENSE file in the root directory of this source tree.
#
import math
import torch
import torch.nn as nn
import torch.nn.functional as F
from fairseq.modules.fairseq_dropout import FairseqDropout
from fairseq.modules.scalar_bias import scalar_bias
class SingleHeadAttention(nn.Module):
"""
Single-head attention that supports Gating and Downsampling
"""
def __init__(
self,
out_channels,
embed_dim,
head_dim,
head_index,
dropout=0.0,
bias=True,
project_input=True,
gated=False,
downsample=False,
num_heads=1,
):
super().__init__()
self.embed_dim = embed_dim
self.dropout_module = FairseqDropout(
dropout, module_name=self.__class__.__name__
)
self.head_index = head_index
self.head_dim = head_dim
self.project_input = project_input
self.gated = gated
self.downsample = downsample
self.num_heads = num_heads
self.projection = None
k_layers = []
v_layers = []
if self.downsample:
k_layers.append(Downsample(self.head_index))
v_layers.append(Downsample(self.head_index))
out_proj_size = self.head_dim
else:
out_proj_size = self.head_dim * self.num_heads
if self.gated:
k_layers.append(GatedLinear(self.embed_dim, out_proj_size, bias=bias))
self.in_proj_q = GatedLinear(self.embed_dim, out_proj_size, bias=bias)
v_layers.append(GatedLinear(self.embed_dim, out_proj_size, bias=bias))
else:
k_layers.append(Linear(self.embed_dim, out_proj_size, bias=bias))
self.in_proj_q = Linear(self.embed_dim, out_proj_size, bias=bias)
v_layers.append(Linear(self.embed_dim, out_proj_size, bias=bias))
self.in_proj_k = nn.Sequential(*k_layers)
self.in_proj_v = nn.Sequential(*v_layers)
if self.downsample:
self.out_proj = Linear(out_proj_size, self.head_dim, bias=bias)
else:
self.out_proj = Linear(out_proj_size, out_channels, bias=bias)
self.scaling = self.head_dim**-0.5
def forward(
self,
query,
key,
value,
mask_future_timesteps=False,
key_padding_mask=None,
use_scalar_bias=False,
):
"""Input shape: Time x Batch x Channel
Self-attention can be implemented by passing in the same arguments for
query, key and value. Future timesteps can be masked with the
`mask_future_timesteps` argument. Padding elements can be excluded from
the key by passing a binary ByteTensor (`key_padding_mask`) with shape:
batch x src_len, where padding elements are indicated by 1s.
"""
src_len, bsz, out_channels = key.size()
tgt_len = query.size(0)
assert list(query.size()) == [tgt_len, bsz, out_channels]
assert key.size() == value.size()
if key_padding_mask is not None:
assert key_padding_mask.size(0) == bsz
assert key_padding_mask.size(1) == src_len
if self.downsample:
size = bsz
else:
size = bsz * self.num_heads
k = key
v = value
q = query
if self.project_input:
q = self.in_proj_q(q)
k = self.in_proj_k(k)
v = self.in_proj_v(v)
src_len = k.size()[0]
q *= self.scaling
if not self.downsample:
q = q.view(tgt_len, size, self.head_dim)
k = k.view(src_len, size, self.head_dim)
v = v.view(src_len, size, self.head_dim)
q = q.transpose(0, 1)
k = k.transpose(0, 1)
v = v.transpose(0, 1)
attn_weights = torch.bmm(q, k.transpose(1, 2))
if mask_future_timesteps:
assert (
query.size() == key.size()
), "mask_future_timesteps only applies to self-attention"
attn_weights *= torch.tril(
attn_weights.data.new([1]).expand(tgt_len, tgt_len).clone(),
diagonal=-1,
)[:, :: self.head_index + 1 if self.downsample else 1].unsqueeze(0)
attn_weights += torch.triu(
attn_weights.data.new([-math.inf]).expand(tgt_len, tgt_len).clone(),
diagonal=0,
)[:, :: self.head_index + 1 if self.downsample else 1].unsqueeze(0)
tgt_size = tgt_len
if use_scalar_bias:
attn_weights = scalar_bias(attn_weights, 2)
v = scalar_bias(v, 1)
tgt_size += 1
if key_padding_mask is not None:
# don't attend to padding symbols
if key_padding_mask.max() > 0:
if self.downsample:
attn_weights = attn_weights.view(bsz, 1, tgt_len, src_len)
else:
attn_weights = attn_weights.view(
size, self.num_heads, tgt_len, src_len
)
attn_weights = attn_weights.masked_fill(
key_padding_mask.unsqueeze(1).unsqueeze(2),
-math.inf,
)
attn_weights = attn_weights.view(size, tgt_len, src_len)
attn_weights = F.softmax(attn_weights, dim=-1)
attn_weights = self.dropout_module(attn_weights)
attn = torch.bmm(attn_weights, v)
if self.downsample:
attn = attn.transpose(0, 1).contiguous().view(tgt_len, bsz, self.head_dim)
else:
attn = attn.transpose(0, 1).contiguous().view(tgt_len, bsz, self.embed_dim)
attn = self.out_proj(attn)
return attn, attn_weights
[docs]class DownsampledMultiHeadAttention(nn.ModuleList):
"""
Multi-headed attention with Gating and Downsampling
"""
def __init__(
self,
out_channels,
embed_dim,
num_heads,
dropout=0.0,
bias=True,
project_input=True,
gated=False,
downsample=False,
):
self.embed_dim = embed_dim
self.num_heads = num_heads
self.head_dim = embed_dim // num_heads
self.downsample = downsample
self.gated = gated
self.project_input = project_input
assert self.head_dim * num_heads == embed_dim
if self.downsample:
attention_heads = []
for index in range(self.num_heads):
attention_heads.append(
SingleHeadAttention(
out_channels,
self.embed_dim,
self.head_dim,
index,
dropout,
bias,
self.project_input,
self.gated,
self.downsample,
self.num_heads,
)
)
super().__init__(modules=attention_heads)
self.out_proj = Linear(embed_dim, out_channels, bias=bias)
else:
# either we have a list of attention heads, or just one attention head
# if not being downsampled, we can do the heads with one linear layer instead of separate ones
super().__init__()
self.attention_module = SingleHeadAttention(
out_channels,
self.embed_dim,
self.head_dim,
1,
dropout,
bias,
self.project_input,
self.gated,
self.downsample,
self.num_heads,
)
[docs] def forward(
self,
query,
key,
value,
mask_future_timesteps=False,
key_padding_mask=None,
use_scalar_bias=False,
):
src_len, bsz, embed_dim = key.size()
tgt_len = query.size(0)
assert embed_dim == self.embed_dim
assert list(query.size()) == [tgt_len, bsz, embed_dim]
assert key.size() == value.size()
tgt_size = tgt_len
if use_scalar_bias:
tgt_size += 1
attn = []
attn_weights = []
if self.downsample:
for attention_head_number in range(self.num_heads):
# call the forward of each attention head
_attn, _attn_weight = self[attention_head_number](
query,
key,
value,
mask_future_timesteps,
key_padding_mask,
use_scalar_bias,
)
attn.append(_attn)
attn_weights.append(_attn_weight)
full_attn = torch.cat(attn, dim=2)
full_attn = self.out_proj(full_attn)
return full_attn, attn_weights[0].clone()
else:
_attn, _attn_weight = self.attention_module(
query,
key,
value,
mask_future_timesteps,
key_padding_mask,
use_scalar_bias,
)
attn.append(_attn)
attn_weights.append(_attn_weight)
full_attn = torch.cat(attn, dim=2)
full_attn_weights = torch.cat(attn_weights)
full_attn_weights = full_attn_weights.view(
bsz, self.num_heads, tgt_size, src_len
)
full_attn_weights = full_attn_weights.sum(dim=1) / self.num_heads
return full_attn, full_attn_weights
class Downsample(nn.Module):
"""
Selects every nth element, where n is the index
"""
def __init__(self, index):
super().__init__()
self.index = index
def forward(self, x):
return x[:: self.index + 1]
def Linear(in_features, out_features, dropout=0.0, bias=True):
"""Weight-normalized Linear layer (input: B x T x C)"""
m = nn.Linear(in_features, out_features, bias=bias)
m.weight.data.normal_(mean=0, std=math.sqrt((1 - dropout) / in_features))
m.bias.data.zero_()
return nn.utils.weight_norm(m)
def GatedLinear(in_features, out_features, dropout=0.0, bias=True):
"""Weight-normalized Linear layer (input: B x T x C) with interspersed GLU units"""
return nn.Sequential(
Linear(in_features, out_features * 4, dropout, bias),
nn.GLU(),
Linear(out_features * 2, out_features * 2, dropout, bias),
nn.GLU(),
Linear(out_features, out_features, dropout, bias),
)