attn_positional.py

# Copyright (c) Facebook, Inc. and its affiliates.
# All rights reserved.
#
# This source code is licensed under the BSD-style license found in the
# LICENSE file in the root directory of this source tree.
import torch
from torch import nn
import math

class BertSelfAttention(nn.Module):
    def __init__(self, hidden_size, num_attention_heads,
                 attention_probs_dropout_prob,
                 position_embedding_type=None, max_position_embeddings=None):
        super().__init__()
        if hidden_size % num_attention_heads != 0:
            raise ValueError(
                f"The hidden size ({hidden_size}) is not a multiple of the number of attention "
                f"heads ({num_attention_heads})"
            )

        self.num_attention_heads = num_attention_heads
        self.attention_head_size = int(hidden_size / num_attention_heads)
        self.all_head_size = self.num_attention_heads * self.attention_head_size

        self.query = nn.Linear(hidden_size, self.all_head_size)
        self.key = nn.Linear(hidden_size, self.all_head_size)
        self.value = nn.Linear(hidden_size, self.all_head_size)

        self.dropout = nn.Dropout(attention_probs_dropout_prob)
        self.position_embedding_type = position_embedding_type

        if self.position_embedding_type is not None:
            assert max_position_embeddings is not None
            self.max_position_embeddings = max_position_embeddings
            self.distance_embedding = nn.Embedding(2 * max_position_embeddings - 1, self.attention_head_size)

    def transpose_for_scores(self, x):
        new_x_shape = x.size()[:-1] + (self.num_attention_heads, self.attention_head_size)
        x = x.view(*new_x_shape)
        return x.permute(0, 2, 1, 3)

    def forward(
        self,
        hidden_states,
        past_key_value=None,
    ):
        q = self.query(hidden_states)
        k = self.key(hidden_states)
        v = self.value(hidden_states)

        q = self.transpose_for_scores(q)
        k = self.transpose_for_scores(k)
        v = self.transpose_for_scores(v)

        if past_key_value is not None:
            k = torch.cat([past_key_value[0], k], dim=2)
            v = torch.cat([past_key_value[1], v], dim=2)


        # Take the dot product between "query" and "key" to get the raw attention scores.
        attention_scores = torch.matmul(q, k.transpose(-1, -2))
        attention_scores = attention_scores / math.sqrt(self.attention_head_size)

        if self.position_embedding_type is not None:
            seq_length = hidden_states.size()[1]
            position_ids_l = torch.arange(seq_length, dtype=torch.long, device=hidden_states.device).view(-1, 1)
            position_ids_r = torch.arange(seq_length, dtype=torch.long, device=hidden_states.device).view(1, -1)
            distance = position_ids_l - position_ids_r
            positional_embedding = self.distance_embedding(distance + self.max_position_embeddings - 1)
            positional_embedding = positional_embedding.to(dtype=q.dtype)  # fp16 compatibility

            if self.position_embedding_type == "relative_key":
                relative_position_scores = torch.einsum("bhld,lrd->bhlr", q, positional_embedding)
                attention_scores = attention_scores + relative_position_scores
            elif self.position_embedding_type == "relative_key_query":
                relative_position_scores_query = torch.einsum("bhld,lrd->bhlr", q, positional_embedding)
                relative_position_scores_key = torch.einsum("bhrd,lrd->bhlr", k, positional_embedding)
                attention_scores = attention_scores + relative_position_scores_query + relative_position_scores_key

        attention_probs = attention_scores
        # Normalize the attention scores to probabilities.
        attention_probs = nn.functional.softmax(attention_scores, dim=-1)
        # # This is actually dropping out entire tokens to attend to, which might
        # # seem a bit unusual, but is taken from the original Transformer paper.
        attention_probs = self.dropout(attention_probs)


        context_layer = torch.matmul(attention_probs, v)

        context_layer = context_layer.permute(0, 2, 1, 3).contiguous()
        new_context_layer_shape = context_layer.size()[:-2] + (self.all_head_size,)
        context_layer = context_layer.view(*new_context_layer_shape)
        return context_layer