# 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 List, Optional
import torch
import torch.nn as nn
from fairseq.token_generation_constraints import (
ConstraintState,
OrderedConstraintState,
UnorderedConstraintState,
)
from torch import Tensor
class Search(nn.Module):
def __init__(self, tgt_dict):
super().__init__()
self.pad = tgt_dict.pad()
self.unk = tgt_dict.unk()
self.eos = tgt_dict.eos()
self.vocab_size = len(tgt_dict)
self.src_lengths = torch.tensor(-1)
self.supports_constraints = False
self.stop_on_max_len = False
def step(
self, step, lprobs, scores, prev_output_tokens=None, original_batch_idxs=None
):
"""Take a single search step.
Args:
step: the current search step, starting at 0
lprobs: (bsz x input_beam_size x vocab_size)
the model's log-probabilities over the vocabulary at the current step
scores: (bsz x input_beam_size x step)
the historical model scores of each hypothesis up to this point
prev_output_tokens: (bsz x step)
the previously generated oputput tokens
original_batch_idxs: (bsz)
the tensor with the batch indices, in the range [0, bsz)
this is useful in case there has been applied a re-ordering
and we need to know the orignal indices
Return: A tuple of (scores, indices, beams) where:
scores: (bsz x output_beam_size)
the scores of the chosen elements; output_beam_size can be
larger than input_beam_size, e.g., we may return
2*input_beam_size to account for EOS
indices: (bsz x output_beam_size)
the indices of the chosen elements
beams: (bsz x output_beam_size)
the hypothesis ids of the chosen elements, in the range [0, input_beam_size)
"""
raise NotImplementedError
@torch.jit.export
def set_src_lengths(self, src_lengths):
self.src_lengths = src_lengths
@torch.jit.export
def init_constraints(self, batch_constraints: Optional[Tensor], beam_size: int):
"""Initialize constraint states for constrained decoding (if supported).
Args:
batch_constraints: (torch.Tensor, optional)
the list of constraints, in packed form
beam_size: (int)
the beam size
Returns:
*encoder_out* rearranged according to *new_order*
"""
pass
def prune_sentences(self, batch_idxs: Tensor):
"""
Removes constraint states for completed sentences (if supported).
This is called from sequence_generator._generate() when sentences are
deleted from the batch.
Args:
batch_idxs: Indices of *sentences* whose constraint state should be *kept*.
"""
pass
def update_constraints(self, active_hypos: Tensor):
"""
Updates the constraint states by selecting the beam items that are retained.
This is called at each time step of sequence_generator._generate() when
the set of 2 * {beam_size} candidate hypotheses are reduced to the beam size.
Args:
active_hypos: (batch size, beam size)
list of integers denoting, for each sentence, which beam candidate items
should be kept.
"""
pass
class BeamSearch(Search):
def __init__(self, tgt_dict):
super().__init__(tgt_dict)
self.constraint_states = None
@torch.jit.export
def step(
self,
step: int,
lprobs,
scores: Optional[Tensor],
prev_output_tokens: Optional[Tensor] = None,
original_batch_idxs: Optional[Tensor] = None,
):
bsz, beam_size, vocab_size = lprobs.size()
if step == 0:
# at the first step all hypotheses are equally likely, so use
# only the first beam
lprobs = lprobs[:, ::beam_size, :].contiguous()
else:
# make probs contain cumulative scores for each hypothesis
assert scores is not None
lprobs = lprobs + scores[:, :, step - 1].unsqueeze(-1)
top_prediction = torch.topk(
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,
lprobs.view(bsz, -1).size(1) - 1, # -1 so we never select pad
),
)
scores_buf = top_prediction[0]
indices_buf = top_prediction[1]
# Project back into relative indices and beams
beams_buf = indices_buf // vocab_size
indices_buf = indices_buf.fmod(vocab_size)
# At this point, beams_buf and indices_buf are single-dim and contain relative indices
return scores_buf, indices_buf, beams_buf
class PrefixConstrainedBeamSearch(Search):
def __init__(self, tgt_dict, prefix_allowed_tokens_fn):
super().__init__(tgt_dict)
self.prefix_allowed_tokens_fn = prefix_allowed_tokens_fn
self.stop_on_max_len = True
@torch.jit.export
def apply_mask(self, x, prev_output_tokens, original_batch_idxs):
beam_size = x.shape[0] // original_batch_idxs.shape[0]
original_batch_idxs = (
original_batch_idxs.unsqueeze(-1).repeat((1, beam_size)).flatten().tolist()
)
mask = torch.full_like(x, -math.inf)
for sent_i, (sent, batch_i) in enumerate(
zip(prev_output_tokens, original_batch_idxs)
):
mask[sent_i, :, self.prefix_allowed_tokens_fn(batch_i, sent)] = 0
return mask
@torch.jit.export
def step(
self,
step: int,
lprobs: Tensor,
scores: Tensor,
prev_output_tokens: Tensor,
original_batch_idxs: Tensor,
):
bsz, beam_size, vocab_size = lprobs.size()
lprobs += self.apply_mask(
lprobs.view(bsz * beam_size, 1, vocab_size),
prev_output_tokens,
original_batch_idxs,
).view(bsz, beam_size, vocab_size)
if step == 0:
# at the first step all hypotheses are equally likely, so use
# only the first beam
lprobs = lprobs[:, ::beam_size, :].contiguous()
else:
# make probs contain cumulative scores for each hypothesis
assert scores is not None
lprobs = lprobs + scores[:, :, step - 1].unsqueeze(-1)
top_prediction = torch.topk(
lprobs.view(bsz, -1),
k=min(
# Take the best beam_size predictions. We'll choose the first
# beam_size of these which don't predict eos to continue with.
beam_size,
lprobs.view(bsz, -1).size(1) - 1, # -1 so we never select pad
),
)
scores_buf = top_prediction[0]
indices_buf = top_prediction[1]
beams_buf = indices_buf // vocab_size
indices_buf = indices_buf.fmod(vocab_size)
return scores_buf, indices_buf, beams_buf
class LexicallyConstrainedBeamSearch(Search):
"""Implements lexically constrained beam search as described in
Fast Lexically Constrained Decoding with Dynamic Beam
Allocation for Neural Machine Translation. Post & Vilar,
NAACL 2018. https://www.aclweb.org/anthology/N18-1119/
and
Improved Lexically Constrained Decoding for Translation and
Monolingual Rewriting. Hu et al, NAACL
2019. https://www.aclweb.org/anthology/N19-1090/
This is accomplished by maintaining, for each beam hypothesis, a
ConstraintState object (see constraints.py) that tracks which
constraints have been generated and using this information to
shape the beam for each input sentence.
"""
def __init__(self, tgt_dict, representation):
super().__init__(tgt_dict)
self.representation = representation
self.vocab_size = len(tgt_dict)
self.num_cands = 0
self.supports_constraints = True
@torch.jit.export
def init_constraints(self, batch_constraints: Optional[Tensor], beam_size: int):
self.constraint_states = []
for constraint_tensor in batch_constraints:
if self.representation == "ordered":
constraint_state = OrderedConstraintState.create(constraint_tensor)
elif self.representation == "unordered":
constraint_state = UnorderedConstraintState.create(constraint_tensor)
self.constraint_states.append([constraint_state for i in range(beam_size)])
@torch.jit.export
def prune_sentences(self, batch_idxs: Tensor):
self.constraint_states = [
self.constraint_states[i] for i in batch_idxs.tolist()
]
@torch.jit.export
def update_constraints(self, active_hypos: Tensor):
if self.constraint_states:
batch_size = active_hypos.size(0)
for sentid in range(batch_size):
self.constraint_states[sentid] = [
self.constraint_states[sentid][i] for i in active_hypos[sentid]
]
@torch.jit.export
def step(
self,
step: int,
lprobs: Tensor,
scores: Optional[Tensor],
prev_output_tokens: Optional[Tensor] = None,
original_batch_idxs: Optional[Tensor] = None,
):
"""
A constrained step builds a large candidates list from the following:
- the top 2 * {beam_size} items over the whole beam
- for each item in the beam
- the top {each_k} (default 1)
- all next constraints
We then compute the constrained state of each beam item, and assign
stripe codes: 0 to the best in each bank, 1 to the 2nd-best, and so
on. We then sort by (stripe, score), and truncate the list at
2 * beam size.
Args:
step: the decoder step
lprobs: (batch size, beam size, target vocab)
the target-vocab distributions for each item in the beam.
Retrun: A tuple of (scores, indices, beams, constraints) where:
scores: (batch, output beam size)
the scores of the chosen elements
indices: (batch, output beam size)
the target vocab indices of the chosen elements
beams: (batch, output beam size)
the 0-indexed hypothesis ids of the chosen elements
constraints: (batch, output beam size)
the new constraint states
"""
each_k = 1
device = lprobs.device
batch_size, beam_size, vocab_size = lprobs.size()
self.num_cands = min(
# Just take the k-best. We'll get another k from the 1-best from each
# row, plus more from the constraints
beam_size * 2,
lprobs.view(batch_size, -1).size(1) - 1, # -1 so we never select pad
)
# STEP 0: Preliminary. Prevent EOS for unfinished hyps across all batch items
constraint_states = self.constraint_states
if constraint_states and step > 0:
not_finished_indices = []
for sentno, sent_constraints in enumerate(constraint_states):
for beamno, state in enumerate(sent_constraints):
index = sentno * beam_size + beamno
if not state.finished:
not_finished_indices.append(index)
not_finished_indices = torch.tensor(not_finished_indices)
if not_finished_indices.numel() > 0:
lprobs.view(batch_size * beam_size, -1)[
not_finished_indices, self.eos
] = -math.inf
if step == 0:
# at the first step all hypotheses are equally likely, so use
# only the first beam entry for each batch item
lprobs = lprobs[:, ::beam_size, :].contiguous()
else:
# make probs contain cumulative scores for each hypothesis
assert scores is not None
lprobs = lprobs + scores[:, :, step - 1].unsqueeze(-1)
top_prediction = torch.topk(
lprobs.view(batch_size, -1),
self.num_cands,
)
scores_buf, indices_buf = top_prediction
# Project back into relative indices and beams
beams_buf = indices_buf // vocab_size
indices_buf = indices_buf.fmod(vocab_size)
# Short circuit if there are no constraints in this batch
if not constraint_states:
return scores_buf, indices_buf, beams_buf
# STEP 1: get top-1 from each hypothesis across all sentences in the batch
if step > 0:
top_scores, top_indices = torch.topk(
lprobs.view(batch_size * beam_size, -1),
k=each_k,
dim=1,
)
top_scores = top_scores.view(batch_size, -1)
top_indices = top_indices.view(batch_size, -1)
scores_buf = torch.cat((scores_buf, top_scores), dim=1)
indices_buf = torch.cat((indices_buf, top_indices), dim=1)
new_beams = torch.arange(0, beam_size, device=device).repeat(batch_size, 1)
beams_buf = torch.cat((beams_buf, new_beams), dim=1)
# Now, process sentences in the batch one by one.
new_scores_buf = torch.zeros((batch_size, 2 * beam_size), device=device)
new_indices_buf = torch.zeros((batch_size, 2 * beam_size), device=device).long()
new_beams_buf = torch.zeros((batch_size, 2 * beam_size), device=device).long()
for sentno, states in enumerate(constraint_states):
scores, indices, beams, new_states = self.step_sentence(
step,
sentno,
lprobs[sentno],
constraint_states[sentno],
beams_buf[sentno].clone(),
indices_buf[sentno].clone(),
scores_buf[sentno].clone(),
)
new_scores_buf[sentno] = scores
new_indices_buf[sentno] = indices
new_beams_buf[sentno] = beams
self.constraint_states[sentno] = new_states
return new_scores_buf, new_indices_buf, new_beams_buf
@torch.jit.export
def step_sentence(
self,
step: int,
sentno: int,
lprobs: Tensor,
constraint_states: List[List[ConstraintState]],
beams_buf: Tensor,
indices_buf: Tensor,
scores_buf: Tensor,
):
"""Does per-sentence processing. Adds all constraints for each
hypothesis to the list of candidates; then removes duplicates,
sorts, and dynamically stripes across the banks. All tensor inputs
are collapsed to those pertaining to a single input sentence.
"""
device = lprobs.device
# STEP 2: Add all constraints for each beam item
for beamno, state in enumerate(constraint_states):
next_tokens = torch.tensor(list(state.next_tokens()), device=device).long()
if next_tokens.numel() != 0:
indices_buf = torch.cat((indices_buf, next_tokens))
next_beams = (
torch.tensor(beamno, device=device)
.repeat(next_tokens.size(0))
.long()
)
beams_buf = torch.cat((beams_buf, next_beams))
next_values = lprobs[beamno].take(next_tokens.view(-1))
scores_buf = torch.cat((scores_buf, next_values))
# At the 0th time step, there is just one beam item
if step == 0:
break
# STEP 3: Compute the "bank" for each candidate. This is the
# number of constraints it's generated. We need this so that
# we can do round-robin allocation of the beam across these
# banks. If C is the number of constraints, we select the best
# item in bank C, then the best in bank C-1, etc, followed by
# the 2nd-best in bank C, the 2nd-best in bank C-1, etc, and so
# on, until the maximum beam size. We accomplish this by
# creating a sort key and striping across the banks.
# Compute the new states for all candidates
cands_size = indices_buf.size(0)
constraint_states = [
constraint_states[beams_buf[i]].advance(indices_buf[i])
for i in range(cands_size)
]
banks = torch.tensor([state.bank for state in constraint_states], device=device)
# STEP 4: Sort
num_constraint_tokens = len(state.tokens)
# Sort by keys (bank, score) (i.e., sort banks together, and scores
# within banks). AFAIK pytorch doesn't support either stable sort or
# multi-key sorting, so we have to hack this.
MAX_SCORE = -100
sort_key = (num_constraint_tokens - banks) * MAX_SCORE + scores_buf
sort_values, sort_indices = sort_key.sort(dim=0, descending=True)
scores_buf = scores_buf[sort_indices]
indices_buf = indices_buf[sort_indices]
beams_buf = beams_buf[sort_indices]
banks = banks[sort_indices]
# Sort the constraints to follow suit
constraint_states = [constraint_states[i] for i in sort_indices]
# STEP 5: Remove duplicates. The topk calls (overall and
# per-row) plus the per-row generation of constraints will
# produce duplicates. Here we remove them.
def roll(t):
"""Rolls a 1d tensor left by 1.
[0, 1, 2, 3, 4] becomes [4, 0, 1, 2, 3]
"""
return torch.cat((t[-1].unsqueeze(0), t[0:-1]), dim=0)
# We map candidates (beam, token_id) to a single dimension.
# This is then shifted by 1. We can then easily identify
# duplicates and create a mask that identifies unique
# extensions.
uniques_mask = beams_buf * (self.vocab_size + 1) + indices_buf
uniques_mask = roll(uniques_mask) != uniques_mask
# Use the mask to pare down the data structures
scores_buf = torch.masked_select(scores_buf, uniques_mask)
indices_buf = torch.masked_select(indices_buf, uniques_mask)
beams_buf = torch.masked_select(beams_buf, uniques_mask)
banks = torch.masked_select(banks, uniques_mask)
i = 1
for mask in uniques_mask[1:]:
if not mask:
constraint_states.pop(i)
i += mask
# STEP 6: Assign IDs round-robin across banks, sort, and
# truncate. Now that the candidates are sorted by (bank,
# score) and uniqed, we dynamically allocate the {beam_size}
# beam by striping across the candidates. These stripes will
# be used as sort keys to do round-robin selection. This is
# accomplished in a single pass with offsets. Sorting by
# highest-banks (furthest-along hypotheses) first ensures
# progress through the constraints.
#
# e.g., BANKS: 3 3 3 2 2 2 2 1 1 1 0 0
# OLD STRIPES: 0 1 2 0 1 2 3 0 1 2 0 1
# NEW STRIPES: 0 1+4 2+8 0+1 1+5 2+9 3+11 0+2 1+6 2+10 0+3 1+7
# = 0 5 10 1 6 11 13 2 7 12 3 8
#
# Sorting by this then gives the following banks:
#
# 3 2 1 0 3 2 1 0 3 2 1 2
#
# We'll take the top {beam_size} of these.
stripe_offsets = [offset * (len(banks) + 1) for offset in range(len(banks) + 1)]
stripes = torch.zeros_like(banks)
cur_bank_count = -1
cur_bank = banks[0]
for i, bank in enumerate(banks):
if bank != cur_bank:
cur_bank_count = 0
cur_bank = bank
else:
cur_bank_count += 1
stripes[i] = num_constraint_tokens - bank + stripe_offsets[cur_bank_count]
# STEP 7: Sort by the stripes values
sort_values, sort_indices = stripes.sort(dim=0)
scores_buf = scores_buf[sort_indices]
indices_buf = indices_buf[sort_indices]
beams_buf = beams_buf[sort_indices]
constraint_states = [constraint_states[i] for i in sort_indices]
# STEP 8: Truncate to the candidates size!
scores_buf = scores_buf[: self.num_cands]
indices_buf = indices_buf[: self.num_cands]
beams_buf = beams_buf[: self.num_cands]
return scores_buf, indices_buf, beams_buf, constraint_states
class LengthConstrainedBeamSearch(Search):
def __init__(self, tgt_dict, min_len_a, min_len_b, max_len_a, max_len_b):
super().__init__(tgt_dict)
self.min_len_a = min_len_a
self.min_len_b = min_len_b
self.max_len_a = max_len_a
self.max_len_b = max_len_b
self.beam = BeamSearch(tgt_dict)
self.needs_src_lengths = True
def step(
self,
step: int,
lprobs,
scores,
prev_output_tokens: Optional[Tensor] = None,
original_batch_idxs: Optional[Tensor] = None,
):
min_lens = self.min_len_a * self.src_lengths + self.min_len_b
max_lens = self.max_len_a * self.src_lengths + self.max_len_b
lprobs[step < min_lens, :, self.eos] = -math.inf
lprobs[step >= max_lens, :, self.eos] = 0
return self.beam.step(step, lprobs, scores)
class DiverseBeamSearch(Search):
"""Diverse Beam Search.
See "Diverse Beam Search: Decoding Diverse Solutions from Neural Sequence
Models" for details.
We only implement the Hamming Diversity penalty here, which performed best
in the original paper.
"""
def __init__(self, tgt_dict, num_groups, diversity_strength):
super().__init__(tgt_dict)
self.num_groups = num_groups
self.diversity_strength = -diversity_strength
self.beam = BeamSearch(tgt_dict)
@torch.jit.export
def step(
self,
step: int,
lprobs,
scores,
prev_output_tokens: Optional[Tensor] = None,
original_batch_idxs: Optional[Tensor] = None,
):
bsz, beam_size, vocab_size = lprobs.size()
if beam_size % self.num_groups != 0:
raise ValueError(
"DiverseBeamSearch requires --beam to be divisible by the number of groups"
)
# initialize diversity penalty
diversity_buf = torch.zeros(lprobs[:, 0, :].size()).to(lprobs)
scores_G, indices_G, beams_G = [], [], []
for g in range(self.num_groups):
lprobs_g = lprobs[:, g :: self.num_groups, :]
scores_g = scores[:, g :: self.num_groups, :] if step > 0 else None
# apply diversity penalty
if g > 0:
lprobs_g = torch.add(
lprobs_g,
other=diversity_buf.unsqueeze(1),
alpha=self.diversity_strength,
)
else:
lprobs_g = lprobs_g.contiguous()
scores_buf, indices_buf, beams_buf = self.beam.step(
step, lprobs_g, scores_g
)
beams_buf.mul_(self.num_groups).add_(g)
scores_G.append(scores_buf.clone())
indices_G.append(indices_buf.clone())
beams_G.append(beams_buf.clone())
# update diversity penalty
diversity_buf.scatter_add_(
1, indices_buf, torch.ones(indices_buf.size()).to(diversity_buf)
)
# interleave results from different groups
scores_buf = torch.stack(scores_G, dim=2).view(bsz, -1)
indices_buf = torch.stack(indices_G, dim=2).view(bsz, -1)
beams_buf = torch.stack(beams_G, dim=2).view(bsz, -1)
return scores_buf, indices_buf, beams_buf
class Sampling(Search):
sampling_topk: int
sampling_topp: float
def __init__(self, tgt_dict, sampling_topk=-1, sampling_topp=-1.0):
super().__init__(tgt_dict)
self.sampling_topk = sampling_topk
self.sampling_topp = sampling_topp
def _sample_topp(self, lprobs):
"""Sample among the smallest set of elements whose cumulative probability mass exceeds p.
See `"The Curious Case of Neural Text Degeneration"
(Holtzman et al., 2019) <https://arxiv.org/abs/1904.09751>`_.
Args:
lprobs: (bsz x input_beam_size x vocab_size)
the model's log-probabilities over the vocabulary at the current step
Return: A tuple of (trimed_probs, truncated_indices) where:
trimed_probs: (bsz x input_beam_size x ?)
the model's probabilities over the elements selected to sample from. The
width of the third dimension is determined by top-P.
truncated_indices: (bsz x input_beam_size x ?)
the indices of the chosen elements.
"""
probs = lprobs.exp_()
# sort the last dimension (vocab dimension) in descending order
sorted_probs, sorted_indices = probs.sort(descending=True)
# compute a mask to indicate the words to be included in the top-P set.
cumsum_probs = sorted_probs.cumsum(dim=2)
mask = cumsum_probs.lt(self.sampling_topp)
# note that mask was computed by 'lt'. One more word needs to be included
# so that the cumulative probability mass can exceed p.
cumsum_mask = mask.cumsum(dim=2)
last_included = cumsum_mask[:, :, -1:]
last_included.clamp_(0, mask.size()[2] - 1)
mask = mask.scatter_(2, last_included, 1)
# truncate unnecessary dims.
max_dim = last_included.max()
truncated_mask = mask[:, :, : max_dim + 1]
truncated_probs = sorted_probs[:, :, : max_dim + 1]
truncated_indices = sorted_indices[:, :, : max_dim + 1]
# trim the words that are not in top-P by setting their probabilities
# to 0, so that they would not be sampled later.
trim_mask = ~truncated_mask
trimed_probs = truncated_probs.masked_fill_(trim_mask, 0)
return trimed_probs, truncated_indices
@torch.jit.export
def step(
self,
step: int,
lprobs,
scores,
prev_output_tokens: Optional[Tensor] = None,
original_batch_idxs: Optional[Tensor] = None,
):
bsz, beam_size, vocab_size = lprobs.size()
if step == 0:
# at the first step all hypotheses are equally likely, so use
# only the first beam
lprobs = lprobs[:, ::beam_size, :].contiguous()
if self.sampling_topp > 0:
# only sample from the smallest set of words whose cumulative probability mass exceeds p
probs, top_indices = self._sample_topp(lprobs)
elif self.sampling_topk > 0:
# only sample from top-k candidates
lprobs, top_indices = lprobs.topk(self.sampling_topk)
probs = lprobs.exp_()
else:
probs = lprobs.exp_()
# dummy data to be consistent with true branch for type check
top_indices = torch.empty(0).to(probs)
# sample
if step == 0:
indices_buf = torch.multinomial(
probs.view(bsz, -1),
beam_size,
replacement=True,
).view(bsz, beam_size)
else:
indices_buf = torch.multinomial(
probs.view(bsz * beam_size, -1),
1,
replacement=True,
).view(bsz, beam_size)
if step == 0:
# expand to beam size
probs = probs.expand(bsz, beam_size, -1)
# gather scores
scores_buf = torch.gather(probs, dim=2, index=indices_buf.unsqueeze(-1))
scores_buf = scores_buf.log_().view(bsz, -1)
# remap indices if using top-k or top-P sampling
if self.sampling_topk > 0 or self.sampling_topp > 0:
indices_buf = torch.gather(
top_indices.expand(bsz, beam_size, -1),
dim=2,
index=indices_buf.unsqueeze(-1),
).squeeze(2)
if step == 0:
beams_buf = indices_buf.new_zeros(bsz, beam_size)
else:
beams_buf = torch.arange(0, beam_size).to(indices_buf).repeat(bsz, 1)
# make scores cumulative
scores_buf.add_(
torch.gather(scores[:, :, step - 1], dim=1, index=beams_buf)
)
return scores_buf, indices_buf, beams_buf
class DiverseSiblingsSearch(Search):
"""
Beam search with diverse siblings.
See "A Simple, Fast Diverse Decoding Algorithm for Neural Generation" for details.
https://arxiv.org/abs/1611.08562
1/ Calculate hypotheses for each beam
2/ Intra-sibling ordering
3/ Rewrite scores
4/ Choose top K hypotheses
if diversity_rate == 0 is equivalent to BeamSearch
"""
def __init__(self, tgt_dict, diversity_rate):
super().__init__(tgt_dict)
self.diversity_rate = diversity_rate
self.beam = BeamSearch(tgt_dict)
def step(
self,
step: int,
lprobs,
scores,
prev_output_tokens: Optional[Tensor] = None,
original_batch_idxs: Optional[Tensor] = None,
):
bsz, beam_size, vocab_size = lprobs.size()
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,
lprobs.view(bsz, -1).size(1) - 1, # -1 so we never select pad
)
s_list: List[Tensor]
i_list: List[Tensor]
s_list = [torch.empty(0).to(lprobs) for i in range(beam_size)]
i_list = [torch.LongTensor().to(device=lprobs.device) for i in range(beam_size)]
sibling_score = torch.arange(1, k + 1).to(lprobs) * self.diversity_rate
if step == 0:
return self.beam.step(step, lprobs, scores)
lprobs.add_(scores[:, :, step - 1].unsqueeze(-1))
# 1/ Calculate hypotheses for each beam
for i in range(beam_size):
torch.topk(lprobs[:, i, :].view(bsz, -1), k, out=(s_list[i], i_list[i]))
i_list[i].fmod_(vocab_size)
# 2/ Intra-sibling ordering by default from topk + 3/ Rewrite scores
s_list[i].sub_(sibling_score)
# 4/ Choose top K hypotheses
indices = torch.stack(i_list, dim=1).view(bsz, -1)
final_scores = torch.empty(0).to(lprobs)
final_indices = torch.LongTensor().to(device=lprobs.device)
final_beams = torch.LongTensor().to(device=lprobs.device)
(final_scores, final_indices) = torch.topk(
torch.stack(s_list, dim=1).view(bsz, -1),
k,
)
final_beams = final_indices // k
for i in range(bsz):
final_indices[i] = indices[i][final_indices[i]]
return final_scores, final_indices, final_beams