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# 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 torch
from fairseq.search import Search
class NoisyChannelBeamSearch(Search):
def __init__(self, tgt_dict):
super().__init__(tgt_dict)
self.fw_scores_buf = None
self.lm_scores_buf = None
def _init_buffers(self, t):
# super()._init_buffers(t)
if self.fw_scores_buf is None:
self.scores_buf = t.new()
self.indices_buf = torch.LongTensor().to(device=t.device)
self.beams_buf = torch.LongTensor().to(device=t.device)
self.fw_scores_buf = t.new()
self.lm_scores_buf = t.new()
def combine_fw_bw(self, combine_method, fw_cum, bw, step):
if combine_method == "noisy_channel":
fw_norm = fw_cum.div(step + 1)
lprobs = bw + fw_norm
elif combine_method == "lm_only":
lprobs = bw + fw_cum
return lprobs
def step(self, step, fw_lprobs, scores, bw_lprobs, lm_lprobs, combine_method):
self._init_buffers(fw_lprobs)
bsz, beam_size, vocab_size = fw_lprobs.size()
if step == 0:
# at the first step all hypotheses are equally likely, so use
# only the first beam
fw_lprobs = fw_lprobs[:, ::beam_size, :].contiguous()
bw_lprobs = bw_lprobs[:, ::beam_size, :].contiguous()
# nothing to add since we are at the first step
fw_lprobs_cum = fw_lprobs
else:
# make probs contain cumulative scores for each hypothesis
raw_scores = (scores[:, :, step - 1].unsqueeze(-1))
fw_lprobs_cum = (fw_lprobs.add(raw_scores))
combined_lprobs = self.combine_fw_bw(combine_method, fw_lprobs_cum, bw_lprobs, step)
# choose the top k according to the combined noisy channel model score
torch.topk(
combined_lprobs.view(bsz, -1),
k=min(
# Take the best 2 x beam_size predictions. We'll choose the first
# beam_size of these which don't predict eos to continue with.
beam_size * 2,
combined_lprobs.view(bsz, -1).size(1) - 1, # -1 so we never select pad
),
out=(self.scores_buf, self.indices_buf),
)
# save corresponding fw and lm scores
self.fw_scores_buf = torch.gather(fw_lprobs_cum.view(bsz, -1), 1, self.indices_buf)
self.lm_scores_buf = torch.gather(lm_lprobs.view(bsz, -1), 1, self.indices_buf)
# Project back into relative indices and beams
self.beams_buf = self.indices_buf // vocab_size
self.indices_buf.fmod_(vocab_size)
return self.scores_buf, self.fw_scores_buf, self.lm_scores_buf, self.indices_buf, self.beams_buf
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