character token embeddings for word level predictions

fairseq/models/transformer.py

@@ -15,7 +15,8 @@ from fairseq import options
 from fairseq import utils
 
 from fairseq.modules import (
-    AdaptiveSoftmax, LearnedPositionalEmbedding, MultiheadAttention, SinusoidalPositionalEmbedding
+    AdaptiveSoftmax, CharacterTokenEmbedder, LearnedPositionalEmbedding, MultiheadAttention,
+    SinusoidalPositionalEmbedding
 )
 
 from . import (
@@ -161,6 +162,15 @@ class TransformerLanguageModel(FairseqLanguageModel):
                             help='if set, disables positional embeddings (outside self attention)')
         parser.add_argument('--share-decoder-input-output-embed', default=False, action='store_true',
                             help='share decoder input and output embeddings')
+        parser.add_argument('--character-embeddings', default=False, action='store_true',
+                            help='if set, uses character embedding convolutions to produce token embeddings')
+        parser.add_argument('--character-filters', type=str, metavar='LIST',
+                            default='[(1, 64), (2, 128), (3, 192), (4, 256), (5, 256), (6, 256), (7, 256)]',
+                            help='size of character embeddings')
+        parser.add_argument('--character-embedding-dim', type=int, metavar='N', default=4,
+                            help='size of character embeddings')
+        parser.add_argument('--char-embedder-highway-layers', type=int, metavar='N', default=2,
+                            help='number of highway layers for character token embeddder')
 
     @classmethod
     def build_model(cls, args, task):
@@ -174,7 +184,19 @@ class TransformerLanguageModel(FairseqLanguageModel):
         if not hasattr(args, 'max_target_positions'):
             args.max_target_positions = args.tokens_per_sample
 
-        embed_tokens = Embedding(len(task.dictionary), args.decoder_embed_dim, task.dictionary.pad())
+        if args.character_embeddings:
+            if not hasattr(args, 'char_embedder_highway_layers'):
+                args.char_embedder_highway_layers = 0
+            if not hasattr(args, 'character_filters'):
+                args.character_filters = '[(1, 4), (2, 8), (3, 16), (4, 32), (5, 64)]'
+
+            embed_tokens = CharacterTokenEmbedder(task.dictionary, eval(args.character_filters),
+                                                  args.character_embedding_dim,
+                                                  args.decoder_embed_dim,
+                                                  args.char_embedder_highway_layers,
+                                                  )
+        else:
+            embed_tokens = Embedding(len(task.dictionary), args.decoder_embed_dim, task.dictionary.pad())
 
         decoder = TransformerDecoder(args, task.dictionary, embed_tokens, no_encoder_attn=True)
         return TransformerLanguageModel(decoder)

fairseq/modules/__init__.py

@@ -7,9 +7,11 @@
 
 from .adaptive_softmax import AdaptiveSoftmax
 from .beamable_mm import BeamableMM
+from .character_token_embedder import CharacterTokenEmbedder
 from .conv_tbc import ConvTBC
 from .downsampled_multihead_attention import DownsampledMultiHeadAttention
 from .grad_multiply import GradMultiply
+from .highway import Highway
 from .learned_positional_embedding import LearnedPositionalEmbedding
 from .linearized_convolution import LinearizedConvolution
 from .multihead_attention import MultiheadAttention
@@ -19,9 +21,11 @@ from .sinusoidal_positional_embedding import SinusoidalPositionalEmbedding
 __all__ = [
     'AdaptiveSoftmax',
     'BeamableMM',
+    'CharacterTokenEmbedder',
     'ConvTBC',
     'DownsampledMultiHeadAttention',
     'GradMultiply',
+    'Highway',
     'LearnedPositionalEmbedding',
     'LinearizedConvolution',
     'MultiheadAttention',

fairseq/modules/character_token_embedder.py

@@ -0,0 +1,126 @@
+# 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 numpy as np
+import torch
+import torch.nn.functional as F
+
+from torch import nn
+from torch.nn.utils.rnn import pad_sequence
+
+from typing import List, Tuple
+
+from .highway import Highway
+from fairseq.data import Dictionary
+
+
+class CharacterTokenEmbedder(torch.nn.Module):
+    def __init__(
+            self,
+            vocab: Dictionary,
+            filters: List[Tuple[int, int]],
+            char_embed_dim: int,
+            word_embed_dim: int,
+            highway_layers: int,
+            max_char_len: int = 50,
+    ):
+        super(CharacterTokenEmbedder, self).__init__()
+
+        self.embedding_dim = word_embed_dim
+        self.char_embeddings = nn.Embedding(257, char_embed_dim, padding_idx=0)
+        self.symbol_embeddings = nn.Parameter(torch.FloatTensor(2, word_embed_dim))
+        self.eos_idx, self.unk_idx = 0, 1
+
+        self.convolutions = nn.ModuleList()
+        for width, out_c in filters:
+            self.convolutions.append(
+                nn.Conv1d(char_embed_dim, out_c, kernel_size=width)
+            )
+
+        final_dim = sum(f[1] for f in filters)
+
+        self.highway = Highway(final_dim, highway_layers)
+        self.projection = nn.Linear(final_dim, word_embed_dim)
+
+        self.set_vocab(vocab, max_char_len)
+        self.reset_parameters()
+
+    def set_vocab(self, vocab, max_char_len):
+        word_to_char = torch.LongTensor(len(vocab), max_char_len)
+
+        truncated = 0
+        for i in range(len(vocab)):
+            if i < vocab.nspecial:
+                char_idxs = [0] * max_char_len
+            else:
+                chars = vocab[i].encode()
+                # +1 for padding
+                char_idxs = [c + 1 for c in chars] + [0] * (max_char_len - len(chars))
+            if len(char_idxs) > max_char_len:
+                truncated += 1
+                char_idxs = char_idxs[:max_char_len]
+            word_to_char[i] = torch.LongTensor(char_idxs)
+
+        if truncated > 0:
+            print('Truncated {} words longer than {} characters'.format(truncated, max_char_len))
+
+        self.vocab = vocab
+        self.word_to_char = word_to_char
+
+    @property
+    def padding_idx(self):
+        return self.vocab.pad()
+
+    def reset_parameters(self):
+        nn.init.xavier_normal_(self.char_embeddings.weight)
+        nn.init.xavier_normal_(self.symbol_embeddings)
+        nn.init.xavier_normal_(self.projection.weight)
+        nn.init.constant_(self.char_embeddings.weight[self.char_embeddings.padding_idx], 0.)
+        nn.init.constant_(self.projection.bias, 0.)
+
+    def forward(
+            self,
+            words: torch.Tensor,
+    ):
+        self.word_to_char = self.word_to_char.type_as(words)
+
+        flat_words = words.view(-1)
+        word_embs = self._convolve(self.word_to_char[flat_words])
+
+        pads = flat_words.eq(self.vocab.pad())
+        if pads.any():
+            word_embs[pads] = 0
+
+        eos = flat_words.eq(self.vocab.eos())
+        if eos.any():
+            word_embs[eos] = self.symbol_embeddings[self.eos_idx]
+
+        unk = flat_words.eq(self.vocab.unk())
+        if unk.any():
+            word_embs[unk] = self.symbol_embeddings[self.unk_idx]
+
+        return word_embs.view(words.size() + (-1,))
+
+    def _convolve(
+            self,
+            char_idxs: torch.Tensor,
+    ):
+        char_embs = self.char_embeddings(char_idxs)
+        char_embs = char_embs.transpose(1, 2)  # BTC -> BCT
+
+        conv_result = []
+
+        for i, conv in enumerate(self.convolutions):
+            x = conv(char_embs)
+            x, _ = torch.max(x, -1)
+            x = F.relu(x)
+            conv_result.append(x)
+
+        conv_result = torch.cat(conv_result, dim=-1)
+        conv_result = self.highway(conv_result)
+
+        return self.projection(conv_result)

fairseq/modules/highway.py

@@ -0,0 +1,55 @@
+# 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 torch.nn.functional as F
+
+from torch import nn
+
+
+class Highway(torch.nn.Module):
+    """
+    A `Highway layer <https://arxiv.org/abs/1505.00387>
+    Adopted from the AllenNLP implementation
+    """
+
+    def __init__(
+            self,
+            input_dim: int,
+            num_layers: int = 1
+    ):
+        super(Highway, self).__init__()
+        self.input_dim = input_dim
+        self.layers = nn.ModuleList([nn.Linear(input_dim, input_dim * 2)
+                                     for _ in range(num_layers)])
+        self.activation = nn.ReLU()
+
+        self.reset_parameters()
+
+    def reset_parameters(self):
+        for layer in self.layers:
+            # As per comment in AllenNLP:
+            # We should bias the highway layer to just carry its input forward.  We do that by
+            # setting the bias on `B(x)` to be positive, because that means `g` will be biased to
+            # be high, so we will carry the input forward.  The bias on `B(x)` is the second half
+            # of the bias vector in each Linear layer.
+            nn.init.constant_(layer.bias[self.input_dim:], 1)
+
+            nn.init.constant_(layer.bias[:self.input_dim], 0)
+            nn.init.xavier_normal_(layer.weight)
+
+    def forward(
+            self,
+            x: torch.Tensor
+    ):
+        for layer in self.layers:
+            projection = layer(x)
+            proj_x, gate = projection.chunk(2, dim=-1)
+            proj_x = self.activation(proj_x)
+            gate = F.sigmoid(gate)
+            x = gate * x + (1 - gate) * proj_x
+        return x

tests/test_character_token_embedder.py

@@ -0,0 +1,44 @@
+# 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.data import Dictionary
+from fairseq.modules import CharacterTokenEmbedder
+
+
+class TestCharacterTokenEmbedder(unittest.TestCase):
+    def test_character_token_embedder(self):
+        vocab = Dictionary()
+        vocab.add_symbol('hello')
+        vocab.add_symbol('there')
+
+        embedder = CharacterTokenEmbedder(vocab, [(2, 16), (4, 32), (8, 64), (16, 2)], 64, 5)
+
+        test_sents = [['hello', 'unk', 'there'], ['there'], ['hello', 'there']]
+        max_len = max(len(s) for s in test_sents)
+        input = torch.LongTensor(len(test_sents), max_len + 2)
+        for i in range(len(test_sents)):
+            input[i][0] = vocab.eos()
+            for j in range(len(test_sents[i])):
+                input[i][j + 1] = vocab.index(test_sents[i][j])
+            input[i][j + 2] = vocab.eos()
+        embs = embedder(input)
+
+        assert embs.size() == (len(test_sents), max_len + 2, 5)
+        assert embs[0][0].equal(embs[1][0])
+        assert embs[0][0].equal(embs[0][-1])
+        assert embs[0][1].equal(embs[2][1])
+        assert embs[0][3].equal(embs[1][1])
+
+        embs.sum().backward()
+        assert embedder.char_embeddings.weight.grad is not None
+
+
+if __name__ == '__main__':
+    unittest.main()