Implement sparse transformer fixed attention pattern (#804)

Summary:
Pull Request resolved: facebookresearch/pytext#804

Pull Request resolved: fairinternal/fairseq-py#746

Pull Request resolved: #894

Adding an implementation of the sparse transformer to multi-head attention using the fixed attention pattern specified https://arxiv.org/pdf/1904.10509.pdf. The sparse_mask masks out words using -inf; after softmax, -inf becomes 0. Thus, a mask does not need to be re-calculated and re-applied when multiplying attn_weights and values.

Four inputs are added to the config: sparse, is_bidirectional, stride, expressivity. If we are using the sparse transformer, is_bidirectional, stride, and expressivity must be specified (there are defaults). If is_bidirectional is False, the mask values using the fixed attention pattern described in the paper. If is_bidirectional is True, subset one includes all values in the current stride window and a summary from every stride window--all other values are masked. Stride (L in the paper) controls the window size and expressivity (c in the paper) controls the size of the summary.

Reviewed By: borguz

Differential Revision: D16042988

fbshipit-source-id: c59166dc7cfe89187a256e4076000c2458842fd5

Author: Sara Hanson

fairseq/modules/multihead_attention.py

@@ -40,7 +40,6 @@ def __init__(self, embed_dim, num_heads, kdim=None, vdim=None, dropout=0., bias=
         assert not self.self_attention or self.qkv_same_dim, 'Self-attention requires query, key and ' \
                                                              'value to be of the same size'
 
-
         if self.qkv_same_dim:
             self.in_proj_weight = Parameter(torch.Tensor(3 * embed_dim, embed_dim))
         else:
@@ -102,7 +101,6 @@ def forward(self, query, key, value, key_padding_mask=None, incremental_state=No
         the key by passing a binary ByteTensor (`key_padding_mask`) with shape:
         batch x src_len, where padding elements are indicated by 1s.
         """
-
         tgt_len, bsz, embed_dim = query.size()
         assert embed_dim == self.embed_dim
         assert list(query.size()) == [tgt_len, bsz, embed_dim]
@@ -217,6 +215,8 @@ def forward(self, query, key, value, key_padding_mask=None, incremental_state=No
                     [key_padding_mask, torch.zeros(key_padding_mask.size(0), 1).type_as(key_padding_mask)], dim=1)
 
         attn_weights = torch.bmm(q, k.transpose(1, 2))
+        attn_weights = self.apply_sparse_mask(attn_weights, tgt_len, src_len, bsz)
+
         assert list(attn_weights.size()) == [bsz * self.num_heads, tgt_len, src_len]
 
         if attn_mask is not None:
@@ -327,3 +327,6 @@ def _set_input_buffer(self, incremental_state, buffer):
             'attn_state',
             buffer,
         )
+
+    def apply_sparse_mask(self, attn_weights, tgt_len, src_len, bsz):
+        return attn_weights

fairseq/modules/sparse_multihead_attention.py

@@ -0,0 +1,106 @@
+# Copyright (c) 2017-present, Facebook, Inc.
+# All rights reserved.
+#
+# This source code is licensed under the license found in the LICENSE file in
+# the root directory of this source tree. An additional grant of patent rights
+# can be found in the PATENTS file in the same directory.
+
+import math
+import torch
+from .multihead_attention import MultiheadAttention
+
+
+class SparseMultiheadAttention(MultiheadAttention):
+    """ Sparse Multi-Headed Attention.
+
+    "Generating Long Sequences with Sparse Transformers". Implements
+    fixed factorized self attention, where l=stride and c=expressivity.
+    A(1) includes all words in the stride window and A(2) takes a summary of c
+    words from the end of each stride window.
+    If is_bidirectional=False, we do not include any words past the current word,
+    as in the paper.
+    """
+
+    def __init__(self, embed_dim, num_heads, kdim=None, vdim=None, dropout=0., bias=True,
+                 add_bias_kv=False, add_zero_attn=False, self_attention=False,
+                 encoder_decoder_attention=False, stride=32, expressivity=8, is_bidirectional=True):
+
+        super().__init__(
+            embed_dim, num_heads, kdim, vdim, dropout, bias, add_bias_kv,
+            add_zero_attn, self_attention, encoder_decoder_attention
+        )
+
+        self.is_bidirectional = is_bidirectional
+        self.stride = stride
+        self.expressivity = expressivity
+        assert(self.stride > 0 and self.stride >= self.expressivity)
+
+    # Used for Ai(2) calculations - beginning of [l-c, l] range
+    def compute_checkpoint(self, word_index):
+        if word_index % self.stride == 0 and word_index is not 0:
+            checkpoint_index = word_index - self.expressivity
+        else:
+            checkpoint_index = (
+                math.floor(word_index / self.stride) * self.stride
+                + self.stride - self.expressivity
+            )
+        return checkpoint_index
+
+    # Computes Ai(2)
+    def compute_subset_summaries(self, absolute_max):
+        checkpoint_index = self.compute_checkpoint(0)
+        subset_two = set()
+        while checkpoint_index <= absolute_max-1:
+            summary = set(range(checkpoint_index, min(
+                checkpoint_index+self.expressivity+1, absolute_max)
+            ))
+            subset_two = subset_two.union(summary)
+            checkpoint_index = self.compute_checkpoint(checkpoint_index+self.stride)
+        return subset_two
+
+    # Sparse Transformer Fixed Attention Pattern: https://arxiv.org/pdf/1904.10509.pdf
+    def compute_fixed_attention_subset(self, word_index, tgt_len):
+        # +1s account for range function; [min, max) -> [min, max]
+        if not self.is_bidirectional:
+            absolute_max = word_index + 1
+        else:
+            absolute_max = tgt_len
+
+        # Subset 1 - whole window
+        rounded_index = math.floor((word_index + self.stride) / self.stride) * self.stride
+        if word_index % self.stride == 0 and word_index is not 0:
+            subset_one = set(range(word_index-self.stride, min(absolute_max, word_index+1)))
+        else:
+            subset_one = set(range(max(0, rounded_index - self.stride), min(
+                absolute_max, rounded_index+1))
+            )
+
+        # Subset 2 - summary per window
+        # If bidirectional, subset 2 is the same for every index
+        subset_two = set()
+        if not self.is_bidirectional:
+            subset_two = self.compute_subset_summaries(absolute_max)
+
+        return subset_one.union(subset_two)
+
+    # Compute sparse mask - if bidirectional, can pre-compute and store
+    def buffered_sparse_mask(self, tensor, tgt_len, src_len):
+        assert(tgt_len > self.stride)
+        sparse_mask = torch.empty((tgt_len, src_len)).float().fill_(float('-inf'))
+
+        # If bidirectional, subset 2 is the same for every index
+        subset_summaries = set()
+        if self.is_bidirectional:
+            subset_summaries = self.compute_subset_summaries(tgt_len)
+
+        for i in range(tgt_len):
+            fixed_attention_subset = self.compute_fixed_attention_subset(i, tgt_len)
+            fixed_attention_subset = fixed_attention_subset.union(subset_summaries)
+            included_word_indices = torch.LongTensor(list(fixed_attention_subset))
+            sparse_mask[i].index_fill_(0, included_word_indices, 0)
+        return sparse_mask.type_as(tensor)
+
+    def apply_sparse_mask(self, attn_weights, tgt_len, src_len, bsz):
+        sparse_mask = self.buffered_sparse_mask(attn_weights, tgt_len, src_len)
+        sparse_mask = sparse_mask.unsqueeze(0).expand(bsz * self.num_heads, tgt_len, src_len)
+        attn_weights += sparse_mask

fairseq/modules/sparse_transformer_sentence_encoder.py

@@ -0,0 +1,85 @@
+# Copyright (c) 2017-present, Facebook, Inc.
+# All rights reserved.
+#
+# This source code is licensed under the license found in the LICENSE file in
+# the root directory of this source tree. An additional grant of patent rights
+# can be found in the PATENTS file in the same directory.
+
+import torch.nn as nn
+from fairseq.modules import TransformerSentenceEncoder
+from fairseq.modules.sparse_transformer_sentence_encoder_layer import SparseTransformerSentenceEncoderLayer
+
+
+class SparseTransformerSentenceEncoder(TransformerSentenceEncoder):
+    """
+    Sparse implementation of the TransformerSentenceEncoder
+    - see SparseMultiheadAttention
+    """
+
+    def __init__(
+        self,
+        padding_idx: int,
+        vocab_size: int,
+        num_encoder_layers: int = 6,
+        embedding_dim: int = 768,
+        ffn_embedding_dim: int = 3072,
+        num_attention_heads: int = 8,
+        dropout: float = 0.1,
+        attention_dropout: float = 0.1,
+        activation_dropout: float = 0.1,
+        max_seq_len: int = 256,
+        num_segments: int = 2,
+        use_position_embeddings: bool = True,
+        offset_positions_by_padding: bool = True,
+        encoder_normalize_before: bool = False,
+        apply_bert_init: bool = False,
+        activation_fn: str = "relu",
+        learned_pos_embedding: bool = True,
+        add_bias_kv: bool = False,
+        add_zero_attn: bool = False,
+        embed_scale: float = None,
+        freeze_embeddings: bool = False,
+        n_trans_layers_to_freeze: int = 0,
+        export: bool = False,
+        is_bidirectional: bool = True,
+        stride: int = 32,
+        expressivity: int = 8,
+    ) -> None:
+
+        super().__init__(
+            padding_idx, vocab_size, num_encoder_layers, embedding_dim,
+            ffn_embedding_dim, num_attention_heads, dropout, attention_dropout,
+            activation_dropout, max_seq_len, num_segments, use_position_embeddings,
+            offset_positions_by_padding, encoder_normalize_before, apply_bert_init,
+            activation_fn, learned_pos_embedding, add_bias_kv, add_zero_attn,
+            embed_scale, freeze_embeddings, n_trans_layers_to_freeze, export
+        )
+
+        self.layers = nn.ModuleList(
+            [
+                SparseTransformerSentenceEncoderLayer(
+                    embedding_dim=self.embedding_dim,
+                    ffn_embedding_dim=ffn_embedding_dim,
+                    num_attention_heads=num_attention_heads,
+                    dropout=self.dropout,
+                    attention_dropout=attention_dropout,
+                    activation_dropout=activation_dropout,
+                    activation_fn=activation_fn,
+                    add_bias_kv=add_bias_kv,
+                    add_zero_attn=add_zero_attn,
+                    export=export,
+                    is_bidirectional=is_bidirectional,
+                    stride=stride,
+                    expressivity=expressivity,
+                )
+                for _ in range(num_encoder_layers)
+            ]
+        )
+
+        def freeze_module_params(m):
+            if m is not None:
+                for p in m.parameters():
+                    p.requires_grad = False
+
+        for layer in range(n_trans_layers_to_freeze):
+            freeze_module_params(self.layers[layer])

fairseq/modules/sparse_transformer_sentence_encoder_layer.py

@@ -0,0 +1,50 @@
+# Copyright (c) 2017-present, Facebook, Inc.
+# All rights reserved.
+#
+# This source code is licensed under the license found in the LICENSE file in
+# the root directory of this source tree. An additional grant of patent rights
+# can be found in the PATENTS file in the same directory.
+
+from fairseq.modules import TransformerSentenceEncoderLayer
+from fairseq.modules.sparse_multihead_attention import SparseMultiheadAttention
+
+
+class SparseTransformerSentenceEncoderLayer(TransformerSentenceEncoderLayer):
+    """
+    Implements a Sprase Transformer Encoder Layer (see SparseMultiheadAttention)
+    """
+
+    def __init__(
+        self,
+        embedding_dim: float = 768,
+        ffn_embedding_dim: float = 3072,
+        num_attention_heads: float = 8,
+        dropout: float = 0.1,
+        attention_dropout: float = 0.1,
+        activation_dropout: float = 0.1,
+        activation_fn: str = 'relu',
+        add_bias_kv: bool = False,
+        add_zero_attn: bool = False,
+        export: bool = False,
+        is_bidirectional: bool = True,
+        stride: int = 32,
+        expressivity: int = 8,
+    ) -> None:
+
+        super().__init__(
+            embedding_dim, ffn_embedding_dim, num_attention_heads, dropout,
+            attention_dropout, activation_dropout, activation_fn, add_bias_kv,
+            add_zero_attn, export
+        )
+
+        self.self_attn = SparseMultiheadAttention(
+            self.embedding_dim,
+            num_attention_heads,
+            dropout=attention_dropout,
+            add_bias_kv=add_bias_kv,
+            add_zero_attn=add_zero_attn,
+            self_attention=True,
+            is_bidirectional=is_bidirectional,
+            stride=stride,
+            expressivity=expressivity,
+        )

tests/test_sparse_multihead_attention.py

@@ -0,0 +1,50 @@
+# Copyright (c) 2017-present, Facebook, Inc.
+# All rights reserved.
+#
+# This source code is licensed under the license found in the LICENSE file in
+# the root directory of this source tree. An additional grant of patent rights
+# can be found in the PATENTS file in the same directory.
+
+import torch
+import unittest
+from fairseq.modules.sparse_multihead_attention import SparseMultiheadAttention
+
+
+class TestSparseMultiheadAttention(unittest.TestCase):
+    def test_sparse_multihead_attention(self):
+        attn_weights = torch.randn(1, 8, 8)
+        bidirectional_sparse_mask = torch.tensor([
+                [0, 0, 0, 0, 0, float('-inf'), float('-inf'), 0],
+                [0, 0, 0, 0, 0, float('-inf'), float('-inf'), 0],
+                [0, 0, 0, 0, 0, float('-inf'), float('-inf'), 0],
+                [0, 0, 0, 0, 0, float('-inf'), float('-inf'), 0],
+                [float('-inf'), float('-inf'), float('-inf'), 0, 0, 0, 0, 0],
+                [float('-inf'), float('-inf'), float('-inf'), 0, 0, 0, 0, 0],
+                [float('-inf'), float('-inf'), float('-inf'), 0, 0, 0, 0, 0],
+                [float('-inf'), float('-inf'), float('-inf'), 0, 0, 0, 0, 0]
+            ])
+
+        bidirectional_attention = SparseMultiheadAttention(16, 1, stride=4, expressivity=1, is_bidirectional=True)
+        bidirectional_attention_sparse_mask = bidirectional_attention.buffered_sparse_mask(attn_weights, 8, 8)
+        torch.all(torch.eq(bidirectional_attention_sparse_mask, bidirectional_sparse_mask))
+
+        sparse_mask = torch.tensor([
+                [0, float('-inf'), float('-inf'), float('-inf'), float('-inf'), float('-inf'),
+                 float('-inf'), float('-inf')],
+                [0, 0, float('-inf'), float('-inf'), float('-inf'), float('-inf'), float('-inf'), float('-inf')],
+                [0, 0, 0, float('-inf'), float('-inf'), float('-inf'), float('-inf'), float('-inf')],
+                [0, 0, 0, 0, float('-inf'), float('-inf'), float('-inf'), float('-inf')],
+                [0, 0, 0, 0, 0, float('-inf'), float('-inf'), float('-inf')],
+                [float('-inf'), float('-inf'), float('-inf'), 0, 0, 0, float('-inf'), float('-inf')],
+                [float('-inf'), float('-inf'), float('-inf'), 0, 0, 0, 0, float('-inf')],
+                [float('-inf'), float('-inf'), float('-inf'), 0, 0, 0, 0, 0],
+            ])
+
+        attention = SparseMultiheadAttention(16, 1, stride=4, expressivity=1, is_bidirectional=False)
+        attention_sparse_mask = attention.buffered_sparse_mask(attn_weights, 8, 8)
+
+        torch.all(torch.eq(attention_sparse_mask, sparse_mask))
+
+
+if __name__ == '__main__':
+    unittest.main()