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import torch.nn as nn
import torch.nn.functional as F
import torch
import math
import pickle
class Attention(nn.Module):
"""
Compute Scaled Dot Product Attention
"""
def __init__(self):
super().__init__()
def forward(self, query, key, value, mask=None, dropout=None):
d_k = query.size(-1)
scores = torch.matmul(query, key.transpose(-2, -1)) \
/ math.sqrt(d_k)
if mask is not None:
scores = scores.masked_fill(mask == 0, -1e9)
p_attn = F.softmax(scores, dim=-1)
if dropout is not None:
p_attn = dropout(p_attn)
return torch.matmul(p_attn, value), p_attn
class MultiHeadedAttention(nn.Module):
"""
Take in model size and number of heads.
"""
def __init__(self, h, d_model, dropout=0.1):
super().__init__()
assert d_model % h == 0
# We assume d_v always equals d_k
self.d_k = d_model // h
self.h = h
self.linear_layers = nn.ModuleList([nn.Linear(d_model, d_model) for _ in range(3)])
self.output_linear = nn.Linear(d_model, d_model)
self.attention = Attention()
self.dropout = nn.Dropout(p=dropout)
def forward(self, query, key, value, mask=None):
# if mask is not None:
# # Same mask applied to all h heads.
# mask = mask.unsqueeze(1)
nbatches = query.size(0)
# 1) Do all the linear projections in batch from d_model => h x d_k
query, key, value = [l(x).view(nbatches, -1, self.h, self.d_k).transpose(1, 2)
for l, x in zip(self.linear_layers, (query, key, value))]
# 2) Apply attention on all the projected vectors in batch.
x, p_attn = self.attention(query, key, value, mask=mask, dropout=self.dropout)
# 3) "Concat" using a view and apply a final linear.
x = x.transpose(1, 2).contiguous().view(nbatches, -1, self.h * self.d_k)
return self.output_linear(x), p_attn
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