# 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.
import math
from typing import Any, Dict, List, Optional
import torch
import torch.nn as nn
from fairseq import utils
from fairseq.distributed import fsdp_wrap
from fairseq.models import FairseqIncrementalDecoder
from fairseq.models.transformer import TransformerConfig
from fairseq.modules import (
AdaptiveSoftmax,
BaseLayer,
FairseqDropout,
LayerDropModuleList,
LayerNorm,
PositionalEmbedding,
SinusoidalPositionalEmbedding,
transformer_layer,
)
from fairseq.modules.checkpoint_activations import checkpoint_wrapper
from fairseq.modules.quant_noise import quant_noise as apply_quant_noise_
from torch import Tensor
# rewrite name for backward compatibility in `make_generation_fast_`
def module_name_fordropout(module_name: str) -> str:
if module_name == "TransformerDecoderBase":
return "TransformerDecoder"
else:
return module_name
class TransformerDecoderBase(FairseqIncrementalDecoder):
"""
Transformer decoder consisting of *cfg.decoder.layers* layers. Each layer
is a :class:`TransformerDecoderLayer`.
Args:
args (argparse.Namespace): parsed command-line arguments
dictionary (~fairseq.data.Dictionary): decoding dictionary
embed_tokens (torch.nn.Embedding): output embedding
no_encoder_attn (bool, optional): whether to attend to encoder outputs
(default: False).
"""
def __init__(
self,
cfg,
dictionary,
embed_tokens,
no_encoder_attn=False,
output_projection=None,
):
self.cfg = cfg
super().__init__(dictionary)
self.register_buffer("version", torch.Tensor([3]))
self._future_mask = torch.empty(0)
self.dropout_module = FairseqDropout(
cfg.dropout, module_name=module_name_fordropout(self.__class__.__name__)
)
self.decoder_layerdrop = cfg.decoder.layerdrop
self.share_input_output_embed = cfg.share_decoder_input_output_embed
input_embed_dim = embed_tokens.embedding_dim
embed_dim = cfg.decoder.embed_dim
self.embed_dim = embed_dim
self.output_embed_dim = cfg.decoder.output_dim
self.padding_idx = embed_tokens.padding_idx
self.max_target_positions = cfg.max_target_positions
self.embed_tokens = embed_tokens
self.embed_scale = 1.0 if cfg.no_scale_embedding else math.sqrt(embed_dim)
if not cfg.adaptive_input and cfg.quant_noise.pq > 0:
self.quant_noise = apply_quant_noise_(
nn.Linear(embed_dim, embed_dim, bias=False),
cfg.quant_noise.pq,
cfg.quant_noise.pq_block_size,
)
else:
self.quant_noise = None
self.project_in_dim = (
Linear(input_embed_dim, embed_dim, bias=False)
if embed_dim != input_embed_dim
else None
)
self.embed_positions = (
PositionalEmbedding(
self.max_target_positions,
embed_dim,
self.padding_idx,
learned=cfg.decoder.learned_pos,
)
if not cfg.no_token_positional_embeddings
else None
)
if cfg.layernorm_embedding:
self.layernorm_embedding = LayerNorm(embed_dim, export=cfg.export)
else:
self.layernorm_embedding = None
self.cross_self_attention = cfg.cross_self_attention
if self.decoder_layerdrop > 0.0:
self.layers = LayerDropModuleList(p=self.decoder_layerdrop)
else:
self.layers = nn.ModuleList([])
self.layers.extend(
[
self.build_decoder_layer(cfg, no_encoder_attn)
for _ in range(cfg.decoder.layers)
]
)
self.num_layers = len(self.layers)
if cfg.decoder.normalize_before and not cfg.no_decoder_final_norm:
self.layer_norm = LayerNorm(embed_dim, export=cfg.export)
else:
self.layer_norm = None
self.project_out_dim = (
Linear(embed_dim, self.output_embed_dim, bias=False)
if embed_dim != self.output_embed_dim and not cfg.tie_adaptive_weights
else None
)
self.adaptive_softmax = None
self.output_projection = output_projection
if self.output_projection is None:
self.build_output_projection(cfg, dictionary, embed_tokens)
def build_output_projection(self, cfg, dictionary, embed_tokens):
if cfg.adaptive_softmax_cutoff is not None:
self.adaptive_softmax = AdaptiveSoftmax(
len(dictionary),
self.output_embed_dim,
utils.eval_str_list(cfg.adaptive_softmax_cutoff, type=int),
dropout=cfg.adaptive_softmax_dropout,
adaptive_inputs=embed_tokens if cfg.tie_adaptive_weights else None,
factor=cfg.adaptive_softmax_factor,
tie_proj=cfg.tie_adaptive_proj,
)
elif self.share_input_output_embed:
self.output_projection = nn.Linear(
self.embed_tokens.weight.shape[1],
self.embed_tokens.weight.shape[0],
bias=False,
)
self.output_projection.weight = self.embed_tokens.weight
else:
self.output_projection = nn.Linear(
self.output_embed_dim, len(dictionary), bias=False
)
nn.init.normal_(
self.output_projection.weight, mean=0, std=self.output_embed_dim**-0.5
)
num_base_layers = cfg.base_layers
for i in range(num_base_layers):
self.layers.insert(
((i + 1) * cfg.decoder.layers) // (num_base_layers + 1),
BaseLayer(cfg),
)
def build_decoder_layer(self, cfg, no_encoder_attn=False):
layer = transformer_layer.TransformerDecoderLayerBase(cfg, no_encoder_attn)
checkpoint = cfg.checkpoint_activations
if checkpoint:
offload_to_cpu = cfg.offload_activations
layer = checkpoint_wrapper(layer, offload_to_cpu=offload_to_cpu)
# if we are checkpointing, enforce that FSDP always wraps the
# checkpointed layer, regardless of layer size
min_params_to_wrap = cfg.min_params_to_wrap if not checkpoint else 0
layer = fsdp_wrap(layer, min_num_params=min_params_to_wrap)
return layer
def forward(
self,
prev_output_tokens,
encoder_out: Optional[Dict[str, List[Tensor]]] = None,
incremental_state: Optional[Dict[str, Dict[str, Optional[Tensor]]]] = None,
features_only: bool = False,
full_context_alignment: bool = False,
alignment_layer: Optional[int] = None,
alignment_heads: Optional[int] = None,
src_lengths: Optional[Any] = None,
return_all_hiddens: bool = False,
):
"""
Args:
prev_output_tokens (LongTensor): previous decoder outputs of shape
`(batch, tgt_len)`, for teacher forcing
encoder_out (optional): output from the encoder, used for
encoder-side attention, should be of size T x B x C
incremental_state (dict): dictionary used for storing state during
:ref:`Incremental decoding`
features_only (bool, optional): only return features without
applying output layer (default: False).
full_context_alignment (bool, optional): don't apply
auto-regressive mask to self-attention (default: False).
Returns:
tuple:
- the decoder's output of shape `(batch, tgt_len, vocab)`
- a dictionary with any model-specific outputs
"""
x, extra = self.extract_features(
prev_output_tokens,
encoder_out=encoder_out,
incremental_state=incremental_state,
full_context_alignment=full_context_alignment,
alignment_layer=alignment_layer,
alignment_heads=alignment_heads,
)
if not features_only:
x = self.output_layer(x)
return x, extra
def extract_features(
self,
prev_output_tokens,
encoder_out: Optional[Dict[str, List[Tensor]]],
incremental_state: Optional[Dict[str, Dict[str, Optional[Tensor]]]] = None,
full_context_alignment: bool = False,
alignment_layer: Optional[int] = None,
alignment_heads: Optional[int] = None,
):
return self.extract_features_scriptable(
prev_output_tokens,
encoder_out,
incremental_state,
full_context_alignment,
alignment_layer,
alignment_heads,
)
"""
A scriptable subclass of this class has an extract_features method and calls
super().extract_features, but super() is not supported in torchscript. A copy of
this function is made to be used in the subclass instead.
"""
def extract_features_scriptable(
self,
prev_output_tokens,
encoder_out: Optional[Dict[str, List[Tensor]]],
incremental_state: Optional[Dict[str, Dict[str, Optional[Tensor]]]] = None,
full_context_alignment: bool = False,
alignment_layer: Optional[int] = None,
alignment_heads: Optional[int] = None,
):
"""
Similar to *forward* but only return features.
Includes several features from "Jointly Learning to Align and
Translate with Transformer Models" (Garg et al., EMNLP 2019).
Args:
full_context_alignment (bool, optional): don't apply
auto-regressive mask to self-attention (default: False).
alignment_layer (int, optional): return mean alignment over
heads at this layer (default: last layer).
alignment_heads (int, optional): only average alignment over
this many heads (default: all heads).
Returns:
tuple:
- the decoder's features of shape `(batch, tgt_len, embed_dim)`
- a dictionary with any model-specific outputs
"""
bs, slen = prev_output_tokens.size()
if alignment_layer is None:
alignment_layer = self.num_layers - 1
enc: Optional[Tensor] = None
padding_mask: Optional[Tensor] = None
if encoder_out is not None and len(encoder_out["encoder_out"]) > 0:
enc = encoder_out["encoder_out"][0]
if encoder_out is not None and len(encoder_out["encoder_padding_mask"]) > 0:
padding_mask = encoder_out["encoder_padding_mask"][0]
# embed positions
positions = None
if self.embed_positions is not None:
positions = self.embed_positions(
prev_output_tokens, incremental_state=incremental_state
)
if incremental_state is not None:
prev_output_tokens = prev_output_tokens[:, -1:]
if positions is not None:
positions = positions[:, -1:]
# Prevent torchscript exporting issue for dynamic quant embedding
prev_output_tokens = prev_output_tokens.contiguous()
# embed tokens and positions
x = self.embed_scale * self.embed_tokens(prev_output_tokens)
if self.quant_noise is not None:
x = self.quant_noise(x)
if self.project_in_dim is not None:
x = self.project_in_dim(x)
if positions is not None:
x += positions
if self.layernorm_embedding is not None:
x = self.layernorm_embedding(x)
x = self.dropout_module(x)
# B x T x C -> T x B x C
x = x.transpose(0, 1)
self_attn_padding_mask: Optional[Tensor] = None
if self.cross_self_attention or prev_output_tokens.eq(self.padding_idx).any():
self_attn_padding_mask = prev_output_tokens.eq(self.padding_idx)
# decoder layers
attn: Optional[Tensor] = None
inner_states: List[Optional[Tensor]] = [x]
for idx, layer in enumerate(self.layers):
if incremental_state is None and not full_context_alignment:
self_attn_mask = self.buffered_future_mask(x)
else:
self_attn_mask = None
x, layer_attn, _ = layer(
x,
enc,
padding_mask,
incremental_state,
self_attn_mask=self_attn_mask,
self_attn_padding_mask=self_attn_padding_mask,
need_attn=bool((idx == alignment_layer)),
need_head_weights=bool((idx == alignment_layer)),
)
inner_states.append(x)
if layer_attn is not None and idx == alignment_layer:
attn = layer_attn.float().to(x)
if attn is not None:
if alignment_heads is not None:
attn = attn[:alignment_heads]
# average probabilities over heads
attn = attn.mean(dim=0)
if self.layer_norm is not None:
x = self.layer_norm(x)
# T x B x C -> B x T x C
x = x.transpose(0, 1)
if self.project_out_dim is not None:
x = self.project_out_dim(x)
return x, {"attn": [attn], "inner_states": inner_states}
def output_layer(self, features):
"""Project features to the vocabulary size."""
if self.adaptive_softmax is None:
# project back to size of vocabulary
return self.output_projection(features)
else:
return features
def max_positions(self):
"""Maximum output length supported by the decoder."""
if self.embed_positions is None:
return self.max_target_positions
return min(self.max_target_positions, self.embed_positions.max_positions)
def buffered_future_mask(self, tensor):
dim = tensor.size(0)
# self._future_mask.device != tensor.device is not working in TorchScript. This is a workaround.
if (
self._future_mask.size(0) == 0
or (not self._future_mask.device == tensor.device)
or self._future_mask.size(0) < dim
):
self._future_mask = torch.triu(
utils.fill_with_neg_inf(torch.zeros([dim, dim])), 1
)
self._future_mask = self._future_mask.to(tensor)
return self._future_mask[:dim, :dim]
def upgrade_state_dict_named(self, state_dict, name):
"""Upgrade a (possibly old) state dict for new versions of fairseq."""
if isinstance(self.embed_positions, SinusoidalPositionalEmbedding):
weights_key = "{}.embed_positions.weights".format(name)
if weights_key in state_dict:
del state_dict[weights_key]
state_dict[
"{}.embed_positions._float_tensor".format(name)
] = torch.FloatTensor(1)
if f"{name}.output_projection.weight" not in state_dict:
if self.share_input_output_embed:
embed_out_key = f"{name}.embed_tokens.weight"
else:
embed_out_key = f"{name}.embed_out"
if embed_out_key in state_dict:
state_dict[f"{name}.output_projection.weight"] = state_dict[
embed_out_key
]
if not self.share_input_output_embed:
del state_dict[embed_out_key]
for i in range(self.num_layers):
# update layer norms
layer_norm_map = {
"0": "self_attn_layer_norm",
"1": "encoder_attn_layer_norm",
"2": "final_layer_norm",
}
for old, new in layer_norm_map.items():
for m in ("weight", "bias"):
k = "{}.layers.{}.layer_norms.{}.{}".format(name, i, old, m)
if k in state_dict:
state_dict[
"{}.layers.{}.{}.{}".format(name, i, new, m)
] = state_dict[k]
del state_dict[k]
version_key = "{}.version".format(name)
if utils.item(state_dict.get(version_key, torch.Tensor([1]))[0]) <= 2:
# earlier checkpoints did not normalize after the stack of layers
self.layer_norm = None
self.normalize = False
state_dict[version_key] = torch.Tensor([1])
return state_dict
def Linear(in_features, out_features, bias=True):
m = nn.Linear(in_features, out_features, bias)
nn.init.xavier_uniform_(m.weight)
if bias:
nn.init.constant_(m.bias, 0.0)
return m