Source code for fairseq.modules.multihead_attention

# 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 Dict, Optional, Tuple

import torch
import torch.nn.functional as F
from fairseq import utils
from fairseq.incremental_decoding_utils import with_incremental_state
from fairseq.modules.fairseq_dropout import FairseqDropout
from fairseq.modules.quant_noise import quant_noise
from torch import Tensor, nn
from torch.nn import Parameter

[docs]@with_incremental_state class MultiheadAttention(nn.Module): """Multi-headed attention. See "Attention Is All You Need" for more details. """ def __init__( self, embed_dim, num_heads, kdim=None, vdim=None, dropout=0.0, bias=True, add_bias_kv=False, add_zero_attn=False, self_attention=False, encoder_decoder_attention=False, q_noise=0.0, qn_block_size=8, ): super().__init__() self.embed_dim = embed_dim self.kdim = kdim if kdim is not None else embed_dim self.vdim = vdim if vdim is not None else embed_dim self.qkv_same_dim = self.kdim == embed_dim and self.vdim == embed_dim self.num_heads = num_heads self.dropout_module = FairseqDropout( dropout, module_name=self.__class__.__name__ ) self.head_dim = embed_dim // num_heads assert ( self.head_dim * num_heads == self.embed_dim ), "embed_dim must be divisible by num_heads" self.scaling = self.head_dim ** -0.5 self.self_attention = self_attention self.encoder_decoder_attention = encoder_decoder_attention assert not self.self_attention or self.qkv_same_dim, ( "Self-attention requires query, key and " "value to be of the same size" ) self.k_proj = quant_noise( nn.Linear(self.kdim, embed_dim, bias=bias), q_noise, qn_block_size ) self.v_proj = quant_noise( nn.Linear(self.vdim, embed_dim, bias=bias), q_noise, qn_block_size ) self.q_proj = quant_noise( nn.Linear(embed_dim, embed_dim, bias=bias), q_noise, qn_block_size ) self.out_proj = quant_noise( nn.Linear(embed_dim, embed_dim, bias=bias), q_noise, qn_block_size ) if add_bias_kv: self.bias_k = Parameter(torch.Tensor(1, 1, embed_dim)) self.bias_v = Parameter(torch.Tensor(1, 1, embed_dim)) else: self.bias_k = self.bias_v = None self.add_zero_attn = add_zero_attn self.reset_parameters() self.onnx_trace = False self.tpu = False
[docs] def prepare_for_onnx_export_(self): self.onnx_trace = True
[docs] def prepare_for_tpu_(self, **kwargs): self.tpu = True
[docs] def reset_parameters(self): if self.qkv_same_dim: # Empirically observed the convergence to be much better with # the scaled initialization nn.init.xavier_uniform_(self.k_proj.weight, gain=1 / math.sqrt(2)) nn.init.xavier_uniform_(self.v_proj.weight, gain=1 / math.sqrt(2)) nn.init.xavier_uniform_(self.q_proj.weight, gain=1 / math.sqrt(2)) else: nn.init.xavier_uniform_(self.k_proj.weight) nn.init.xavier_uniform_(self.v_proj.weight) nn.init.xavier_uniform_(self.q_proj.weight) nn.init.xavier_uniform_(self.out_proj.weight) if self.out_proj.bias is not None: nn.init.constant_(self.out_proj.bias, 0.0) if self.bias_k is not None: nn.init.xavier_normal_(self.bias_k) if self.bias_v is not None: nn.init.xavier_normal_(self.bias_v)
[docs] def forward( self, query, key: Optional[Tensor], value: Optional[Tensor], key_padding_mask: Optional[Tensor] = None, incremental_state: Optional[Dict[str, Dict[str, Optional[Tensor]]]] = None, need_weights: bool = True, static_kv: bool = False, attn_mask: Optional[Tensor] = None, before_softmax: bool = False, need_head_weights: bool = False, ) -> Tuple[Tensor, Optional[Tensor]]: """Input shape: Time x Batch x Channel Args: key_padding_mask (ByteTensor, optional): mask to exclude keys that are pads, of shape `(batch, src_len)`, where padding elements are indicated by 1s. need_weights (bool, optional): return the attention weights, averaged over heads (default: False). attn_mask (ByteTensor, optional): typically used to implement causal attention, where the mask prevents the attention from looking forward in time (default: None). before_softmax (bool, optional): return the raw attention weights and values before the attention softmax. need_head_weights (bool, optional): return the attention weights for each head. Implies *need_weights*. Default: return the average attention weights over all heads. """ if need_head_weights: need_weights = True tgt_len, bsz, embed_dim = query.size() assert embed_dim == self.embed_dim assert list(query.size()) == [tgt_len, bsz, embed_dim] if ( not self.onnx_trace and not self.tpu # don't use PyTorch version on TPUs and incremental_state is None and not static_kv # A workaround for quantization to work. Otherwise JIT compilation # treats bias in linear module as method. and not torch.jit.is_scripting() ): assert key is not None and value is not None return F.multi_head_attention_forward( query, key, value, self.embed_dim, self.num_heads, torch.empty([0]),, self.k_proj.bias, self.v_proj.bias)), self.bias_k, self.bias_v, self.add_zero_attn, self.dropout_module.p, self.out_proj.weight, self.out_proj.bias, or self.dropout_module.apply_during_inference, key_padding_mask, need_weights, attn_mask, use_separate_proj_weight=True, q_proj_weight=self.q_proj.weight, k_proj_weight=self.k_proj.weight, v_proj_weight=self.v_proj.weight, ) if incremental_state is not None: saved_state = self._get_input_buffer(incremental_state) if saved_state is not None and "prev_key" in saved_state: # previous time steps are cached - no need to recompute # key and value if they are static if static_kv: assert self.encoder_decoder_attention and not self.self_attention key = value = None else: saved_state = None if self.self_attention: q = self.q_proj(query) k = self.k_proj(query) v = self.v_proj(query) elif self.encoder_decoder_attention: # encoder-decoder attention q = self.q_proj(query) if key is None: assert value is None k = v = None else: k = self.k_proj(key) v = self.v_proj(key) else: assert key is not None and value is not None q = self.q_proj(query) k = self.k_proj(key) v = self.v_proj(value) q *= self.scaling if self.bias_k is not None: assert self.bias_v is not None k =[k, self.bias_k.repeat(1, bsz, 1)]) v =[v, self.bias_v.repeat(1, bsz, 1)]) if attn_mask is not None: attn_mask = [attn_mask, attn_mask.new_zeros(attn_mask.size(0), 1)], dim=1 ) if key_padding_mask is not None: key_padding_mask = [ key_padding_mask, key_padding_mask.new_zeros(key_padding_mask.size(0), 1), ], dim=1, ) q = ( q.contiguous() .view(tgt_len, bsz * self.num_heads, self.head_dim) .transpose(0, 1) ) if k is not None: k = ( k.contiguous() .view(-1, bsz * self.num_heads, self.head_dim) .transpose(0, 1) ) if v is not None: v = ( v.contiguous() .view(-1, bsz * self.num_heads, self.head_dim) .transpose(0, 1) ) if saved_state is not None: # saved states are stored with shape (bsz, num_heads, seq_len, head_dim) if "prev_key" in saved_state: _prev_key = saved_state["prev_key"] assert _prev_key is not None prev_key = _prev_key.view(bsz * self.num_heads, -1, self.head_dim) if static_kv: k = prev_key else: assert k is not None k =[prev_key, k], dim=1) if "prev_value" in saved_state: _prev_value = saved_state["prev_value"] assert _prev_value is not None prev_value = _prev_value.view(bsz * self.num_heads, -1, self.head_dim) if static_kv: v = prev_value else: assert v is not None v =[prev_value, v], dim=1) prev_key_padding_mask: Optional[Tensor] = None if "prev_key_padding_mask" in saved_state: prev_key_padding_mask = saved_state["prev_key_padding_mask"] assert k is not None and v is not None key_padding_mask = MultiheadAttention._append_prev_key_padding_mask( key_padding_mask=key_padding_mask, prev_key_padding_mask=prev_key_padding_mask, batch_size=bsz, src_len=k.size(1), static_kv=static_kv, ) saved_state["prev_key"] = k.view(bsz, self.num_heads, -1, self.head_dim) saved_state["prev_value"] = v.view(bsz, self.num_heads, -1, self.head_dim) saved_state["prev_key_padding_mask"] = key_padding_mask # In this branch incremental_state is never None assert incremental_state is not None incremental_state = self._set_input_buffer(incremental_state, saved_state) assert k is not None src_len = k.size(1) # This is part of a workaround to get around fork/join parallelism # not supporting Optional types. if key_padding_mask is not None and key_padding_mask.dim() == 0: key_padding_mask = None if key_padding_mask is not None: assert key_padding_mask.size(0) == bsz assert key_padding_mask.size(1) == src_len if self.add_zero_attn: assert v is not None src_len += 1 k =[k, k.new_zeros((k.size(0), 1) + k.size()[2:])], dim=1) v =[v, v.new_zeros((v.size(0), 1) + v.size()[2:])], dim=1) if attn_mask is not None: attn_mask = [attn_mask, attn_mask.new_zeros(attn_mask.size(0), 1)], dim=1 ) if key_padding_mask is not None: key_padding_mask = [ 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: attn_mask = attn_mask.unsqueeze(0) if self.onnx_trace: attn_mask = attn_mask.repeat(attn_weights.size(0), 1, 1) attn_weights += attn_mask if key_padding_mask is not None: # don't attend to padding symbols attn_weights = attn_weights.view(bsz, self.num_heads, tgt_len, src_len) if not self.tpu: attn_weights = attn_weights.masked_fill( key_padding_mask.unsqueeze(1).unsqueeze(2).to(torch.bool), float("-inf"), ) else: attn_weights = attn_weights.transpose(0, 2) attn_weights = attn_weights.masked_fill(key_padding_mask, float("-inf")) attn_weights = attn_weights.transpose(0, 2) attn_weights = attn_weights.view(bsz * self.num_heads, tgt_len, src_len) if before_softmax: return attn_weights, v attn_weights_float = utils.softmax( attn_weights, dim=-1, onnx_trace=self.onnx_trace ) attn_weights = attn_weights_float.type_as(attn_weights) attn_probs = self.dropout_module(attn_weights) assert v is not None attn = torch.bmm(attn_probs, v) assert list(attn.size()) == [bsz * self.num_heads, tgt_len, self.head_dim] if self.onnx_trace and attn.size(1) == 1: # when ONNX tracing a single decoder step (sequence length == 1) # the transpose is a no-op copy before view, thus unnecessary attn = attn.contiguous().view(tgt_len, bsz, embed_dim) else: attn = attn.transpose(0, 1).contiguous().view(tgt_len, bsz, embed_dim) attn = self.out_proj(attn) attn_weights: Optional[Tensor] = None if need_weights: attn_weights = attn_weights_float.view( bsz, self.num_heads, tgt_len, src_len ).transpose(1, 0) if not need_head_weights: # average attention weights over heads attn_weights = attn_weights.mean(dim=0) return attn, attn_weights
@staticmethod def _append_prev_key_padding_mask( key_padding_mask: Optional[Tensor], prev_key_padding_mask: Optional[Tensor], batch_size: int, src_len: int, static_kv: bool, ) -> Optional[Tensor]: # saved key padding masks have shape (bsz, seq_len) if prev_key_padding_mask is not None and static_kv: new_key_padding_mask = prev_key_padding_mask elif prev_key_padding_mask is not None and key_padding_mask is not None: new_key_padding_mask = [prev_key_padding_mask.float(), key_padding_mask.float()], dim=1 ) # During incremental decoding, as the padding token enters and # leaves the frame, there will be a time when prev or current # is None elif prev_key_padding_mask is not None: filler = torch.zeros( (batch_size, src_len - prev_key_padding_mask.size(1)), device=prev_key_padding_mask.device, ) new_key_padding_mask = [prev_key_padding_mask.float(), filler.float()], dim=1 ) elif key_padding_mask is not None: filler = torch.zeros( (batch_size, src_len - key_padding_mask.size(1)), device=key_padding_mask.device, ) new_key_padding_mask = [filler.float(), key_padding_mask.float()], dim=1 ) else: new_key_padding_mask = prev_key_padding_mask return new_key_padding_mask
[docs] @torch.jit.export def reorder_incremental_state( self, incremental_state: Dict[str, Dict[str, Optional[Tensor]]], new_order: Tensor, ): """Reorder buffered internal state (for incremental generation).""" input_buffer = self._get_input_buffer(incremental_state) if input_buffer is not None: for k in input_buffer.keys(): input_buffer_k = input_buffer[k] if input_buffer_k is not None: if self.encoder_decoder_attention and input_buffer_k.size( 0 ) == new_order.size(0): break input_buffer[k] = input_buffer_k.index_select(0, new_order) incremental_state = self._set_input_buffer(incremental_state, input_buffer) return incremental_state
def _get_input_buffer( self, incremental_state: Optional[Dict[str, Dict[str, Optional[Tensor]]]] ) -> Dict[str, Optional[Tensor]]: result = self.get_incremental_state(incremental_state, "attn_state") if result is not None: return result else: empty_result: Dict[str, Optional[Tensor]] = {} return empty_result def _set_input_buffer( self, incremental_state: Dict[str, Dict[str, Optional[Tensor]]], buffer: Dict[str, Optional[Tensor]], ): return self.set_incremental_state(incremental_state, "attn_state", buffer)
[docs] def apply_sparse_mask(self, attn_weights, tgt_len: int, src_len: int, bsz: int): return attn_weights
[docs] def upgrade_state_dict_named(self, state_dict, name): prefix = name + "." if name != "" else "" items_to_add = {} keys_to_remove = [] for k in state_dict.keys(): if k.endswith(prefix + "in_proj_weight"): # in_proj_weight used to be q + k + v with same dimensions dim = int(state_dict[k].shape[0] / 3) items_to_add[prefix + "q_proj.weight"] = state_dict[k][:dim] items_to_add[prefix + "k_proj.weight"] = state_dict[k][dim : 2 * dim] items_to_add[prefix + "v_proj.weight"] = state_dict[k][2 * dim :] keys_to_remove.append(k) k_bias = prefix + "in_proj_bias" if k_bias in state_dict.keys(): dim = int(state_dict[k].shape[0] / 3) items_to_add[prefix + "q_proj.bias"] = state_dict[k_bias][:dim] items_to_add[prefix + "k_proj.bias"] = state_dict[k_bias][ dim : 2 * dim ] items_to_add[prefix + "v_proj.bias"] = state_dict[k_bias][2 * dim :] keys_to_remove.append(prefix + "in_proj_bias") for k in keys_to_remove: del state_dict[k] for key, value in items_to_add.items(): state_dict[key] = value