# Copyright (c) Facebook, Inc. and its affiliates.
#
# This source code is licensed under the MIT license found in the
# LICENSE file in the root directory of this source tree.
from typing import Optional, Tuple
import torch
import torch.nn as nn
from fairseq.modules import (
FairseqDropout,
LayerDropModuleList,
LayerNorm,
MultiheadAttention,
PositionalEmbedding,
TransformerSentenceEncoderLayer,
)
from fairseq.modules.quant_noise import quant_noise as apply_quant_noise_
def init_bert_params(module):
"""
Initialize the weights specific to the BERT Model.
This overrides the default initializations depending on the specified arguments.
1. If normal_init_linear_weights is set then weights of linear
layer will be initialized using the normal distribution and
bais will be set to the specified value.
2. If normal_init_embed_weights is set then weights of embedding
layer will be initialized using the normal distribution.
3. If normal_init_proj_weights is set then weights of
in_project_weight for MultiHeadAttention initialized using
the normal distribution (to be validated).
"""
if isinstance(module, nn.Linear):
module.weight.data.normal_(mean=0.0, std=0.02)
if module.bias is not None:
module.bias.data.zero_()
if isinstance(module, nn.Embedding):
module.weight.data.normal_(mean=0.0, std=0.02)
if module.padding_idx is not None:
module.weight.data[module.padding_idx].zero_()
if isinstance(module, MultiheadAttention):
module.q_proj.weight.data.normal_(mean=0.0, std=0.02)
module.k_proj.weight.data.normal_(mean=0.0, std=0.02)
module.v_proj.weight.data.normal_(mean=0.0, std=0.02)
class TransformerSentenceEncoder(nn.Module):
"""
Implementation for a Bi-directional Transformer based Sentence Encoder used
in BERT/XLM style pre-trained models.
This first computes the token embedding using the token embedding matrix,
position embeddings (if specified) and segment embeddings
(if specified). After applying the specified number of
TransformerEncoderLayers, it outputs all the internal states of the
encoder as well as the final representation associated with the first
token (usually CLS token).
Input:
- tokens: B x T matrix representing sentences
- segment_labels: B x T matrix representing segment label for tokens
Output:
- a tuple of the following:
- a list of internal model states used to compute the
predictions where each tensor has shape T x B x C
- sentence representation associated with first input token
in format B x C.
"""
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,
layerdrop: float = 0.0,
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,
embed_scale: float = None,
freeze_embeddings: bool = False,
n_trans_layers_to_freeze: int = 0,
export: bool = False,
traceable: bool = False,
q_noise: float = 0.0,
qn_block_size: int = 8,
) -> None:
super().__init__()
self.padding_idx = padding_idx
self.vocab_size = vocab_size
self.dropout_module = FairseqDropout(
dropout, module_name=self.__class__.__name__
)
self.layerdrop = layerdrop
self.max_seq_len = max_seq_len
self.embedding_dim = embedding_dim
self.num_segments = num_segments
self.use_position_embeddings = use_position_embeddings
self.apply_bert_init = apply_bert_init
self.learned_pos_embedding = learned_pos_embedding
self.traceable = traceable
self.embed_tokens = self.build_embedding(
self.vocab_size, self.embedding_dim, self.padding_idx
)
self.embed_scale = embed_scale
if q_noise > 0:
self.quant_noise = apply_quant_noise_(
nn.Linear(self.embedding_dim, self.embedding_dim, bias=False),
q_noise,
qn_block_size,
)
else:
self.quant_noise = None
self.segment_embeddings = (
nn.Embedding(self.num_segments, self.embedding_dim, padding_idx=None)
if self.num_segments > 0
else None
)
self.embed_positions = (
PositionalEmbedding(
self.max_seq_len,
self.embedding_dim,
padding_idx=(self.padding_idx if offset_positions_by_padding else None),
learned=self.learned_pos_embedding,
)
if self.use_position_embeddings
else None
)
if encoder_normalize_before:
self.emb_layer_norm = LayerNorm(self.embedding_dim, export=export)
else:
self.emb_layer_norm = None
if self.layerdrop > 0.0:
self.layers = LayerDropModuleList(p=self.layerdrop)
else:
self.layers = nn.ModuleList([])
self.layers.extend(
[
self.build_transformer_sentence_encoder_layer(
embedding_dim=self.embedding_dim,
ffn_embedding_dim=ffn_embedding_dim,
num_attention_heads=num_attention_heads,
dropout=self.dropout_module.p,
attention_dropout=attention_dropout,
activation_dropout=activation_dropout,
activation_fn=activation_fn,
export=export,
q_noise=q_noise,
qn_block_size=qn_block_size,
)
for _ in range(num_encoder_layers)
]
)
# Apply initialization of model params after building the model
if self.apply_bert_init:
self.apply(init_bert_params)
def freeze_module_params(m):
if m is not None:
for p in m.parameters():
p.requires_grad = False
if freeze_embeddings:
freeze_module_params(self.embed_tokens)
freeze_module_params(self.segment_embeddings)
freeze_module_params(self.embed_positions)
freeze_module_params(self.emb_layer_norm)
for layer in range(n_trans_layers_to_freeze):
freeze_module_params(self.layers[layer])
def build_embedding(self, vocab_size, embedding_dim, padding_idx):
return nn.Embedding(vocab_size, embedding_dim, padding_idx)
def build_transformer_sentence_encoder_layer(
self,
embedding_dim,
ffn_embedding_dim,
num_attention_heads,
dropout,
attention_dropout,
activation_dropout,
activation_fn,
export,
q_noise,
qn_block_size,
):
return TransformerSentenceEncoderLayer(
embedding_dim=embedding_dim,
ffn_embedding_dim=ffn_embedding_dim,
num_attention_heads=num_attention_heads,
dropout=dropout,
attention_dropout=attention_dropout,
activation_dropout=activation_dropout,
activation_fn=activation_fn,
export=export,
q_noise=q_noise,
qn_block_size=qn_block_size,
)
def forward(
self,
tokens: torch.Tensor,
segment_labels: torch.Tensor = None,
last_state_only: bool = False,
positions: Optional[torch.Tensor] = None,
token_embeddings: Optional[torch.Tensor] = None,
attn_mask: Optional[torch.Tensor] = None,
) -> Tuple[torch.Tensor, torch.Tensor]:
is_tpu = tokens.device.type == "xla"
# compute padding mask. This is needed for multi-head attention
padding_mask = tokens.eq(self.padding_idx)
if not self.traceable and not is_tpu and not padding_mask.any():
padding_mask = None
if token_embeddings is not None:
x = token_embeddings
else:
x = self.embed_tokens(tokens)
if self.embed_scale is not None:
x = x * self.embed_scale
if self.embed_positions is not None:
x = x + self.embed_positions(tokens, positions=positions)
if self.segment_embeddings is not None and segment_labels is not None:
x = x + self.segment_embeddings(segment_labels)
if self.quant_noise is not None:
x = self.quant_noise(x)
if self.emb_layer_norm is not None:
x = self.emb_layer_norm(x)
x = self.dropout_module(x)
# account for padding while computing the representation
if padding_mask is not None:
x = x * (1 - padding_mask.unsqueeze(-1).type_as(x))
# B x T x C -> T x B x C
x = x.transpose(0, 1)
inner_states = []
if not last_state_only:
inner_states.append(x)
for layer in self.layers:
x, _ = layer(x, self_attn_padding_mask=padding_mask, self_attn_mask=attn_mask)
if not last_state_only:
inner_states.append(x)
sentence_rep = x[0, :, :]
if last_state_only:
inner_states = [x]
if self.traceable:
return torch.stack(inner_states), sentence_rep
else:
return inner_states, sentence_rep