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https://github.com/labmlai/annotated_deep_learning_paper_implementations.git
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134 lines
4.4 KiB
Python
134 lines
4.4 KiB
Python
"""
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# Multi-Headed Attention
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The implementation is inspired from [Annotated Transformer](https://nlp.seas.harvard.edu/2018/04/03/attention.html)
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"""
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import math
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from typing import Optional
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import torch
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from labml import tracker
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from labml_helpers.module import Module
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from torch import nn as nn
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from torch.nn import functional as F
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class PrepareForMultiHeadAttention(Module):
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"""
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This module does a linear transformation and splits the vector into given
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number of heads for multi-head attention.
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"""
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def __init__(self, d_model: int, heads: int, d_k: int, bias: bool):
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super().__init__()
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self.linear = nn.Linear(d_model, heads * d_k, bias=bias)
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self.heads = heads
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self.d_k = d_k
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def __call__(self, x: torch.Tensor):
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# Input has shape `[seq_len, batch_size, d_model]`
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seq_len, batch_size, _ = x.shape
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x = self.linear(x)
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x = x.view(seq_len, batch_size, self.heads, self.d_k)
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# Output has shape `[seq_len, batch_size, heads, d_k]`
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return x
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class MultiHeadAttention(Module):
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def __init__(self, heads: int, d_model: int, dropout_prob: float = 0.1, bias: bool = True):
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"""
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## Multi-Head Attention Module
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This computes multi-headed attention for given `query`, `key` and `value` vectors.
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`heads` is the number of heads.
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`d_model` is the number of features in the `query`, `key` and `value` vectors.
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$$Attention(Q, K, V) = softmax\Bigg(\frac{Q K^T}{\sqrt{d_k}}\Bigg)V$$
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"""
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super().__init__()
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self.d_k = d_model // heads
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self.heads = heads
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# These transformer the `query`, `key` and `value` vectors for multi-headed attention/
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self.query = PrepareForMultiHeadAttention(d_model, heads, self.d_k, bias)
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self.key = PrepareForMultiHeadAttention(d_model, heads, self.d_k, bias)
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self.value = PrepareForMultiHeadAttention(d_model, heads, self.d_k, bias)
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# Output layer
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self.output = nn.Linear(d_model, d_model)
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self.dropout = nn.Dropout(dropout_prob)
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self.scale = 1 / math.sqrt(self.d_k)
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# We store attentions so that it can used for logging, or other computations if needed
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self.attn = None
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def get_scores(self, query: torch.Tensor, key: torch.Tensor):
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"""
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### Calculate scores between queries and keys.
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This method can be overriden for other variations like relative attention.
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"""
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# Calculate $Q K^T$
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return torch.einsum('ibhd,jbhd->ijbh', query, key)
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def __call__(self, *,
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query: torch.Tensor,
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key: torch.Tensor,
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value: torch.Tensor,
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mask: Optional[torch.Tensor] = None):
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# `query`, `key` and `value` have shape `[seq_len, batch_size, d_model]`
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seq_len, batch_size, _ = query.shape
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if mask is not None:
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# `mask` has shape `[seq_len, seq_len, batch_size]`,
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# where first dimension is the query dimension.
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# If the query dimension is equal to $1$ it will be broadcasted
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assert mask.shape[0] == 1 or mask.shape[0] == mask.shape[1]
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# Same mask applied to all heads.
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mask = mask.unsqueeze(-1)
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# Prepare `query`, `key` and `value` for attention computation
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# These will then have shape `[seq_len, batch_size, heads, d_k]`
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query = self.query(query)
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key = self.key(key)
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value = self.value(value)
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# Compute attention scores $Q K^T$
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# Results in a tensor of shape `[seq_len, seq_len, batch_size, heads]`
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scores = self.get_scores(query, key)
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# Scale scores $\frac{Q K^T}{\sqrt{d_k}}$
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scores *= self.scale
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# Apply mask
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if mask is not None:
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scores = scores.masked_fill(mask == 0, -1e9)
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# $softmax$ attention $softmax\Bigg(\frac{Q K^T}{\sqrt{d_k}}\Bigg)$
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attn = F.softmax(scores, dim=1)
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# Save attentions if debugging
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tracker.debug('attn', attn)
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# Apply dropout
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attn = self.dropout(attn)
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# Multiply by values $softmax\Bigg(\frac{Q K^T}{\sqrt{d_k}}\Bigg)V$
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x = torch.einsum("ijbh,jbhd->ibhd", attn, value)
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# Save attentions for any other calculations
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self.attn = attn.detach()
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# Concatenate multiple heads
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x = x.reshape(seq_len, batch_size, -1)
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# Output layer
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return self.output(x)
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