# 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
import torch
from fairseq import metrics, utils
from fairseq.criterions import FairseqCriterion, register_criterion
def label_smoothed_nll_loss(lprobs, target, epsilon, ignore_index=None, reduce=True):
if target.dim() == lprobs.dim() - 1:
target = target.unsqueeze(-1)
nll_loss = -lprobs.gather(dim=-1, index=target)
smooth_loss = -lprobs.sum(dim=-1, keepdim=True)
if ignore_index is not None:
pad_mask = target.eq(ignore_index)
nll_loss.masked_fill_(pad_mask, 0.0)
smooth_loss.masked_fill_(pad_mask, 0.0)
else:
nll_loss = nll_loss.squeeze(-1)
smooth_loss = smooth_loss.squeeze(-1)
if reduce:
nll_loss = nll_loss.sum()
smooth_loss = smooth_loss.sum()
eps_i = epsilon / lprobs.size(-1)
loss = (1.0 - epsilon) * nll_loss + eps_i * smooth_loss
return loss, nll_loss
[docs]@register_criterion("label_smoothed_cross_entropy")
class LabelSmoothedCrossEntropyCriterion(FairseqCriterion):
def __init__(
self,
task,
sentence_avg,
label_smoothing,
ignore_prefix_size=0,
report_accuracy=False,
):
super().__init__(task)
self.sentence_avg = sentence_avg
self.eps = label_smoothing
self.ignore_prefix_size = ignore_prefix_size
self.report_accuracy = report_accuracy
[docs] @staticmethod
def add_args(parser):
"""Add criterion-specific arguments to the parser."""
# fmt: off
parser.add_argument('--label-smoothing', default=0., type=float, metavar='D',
help='epsilon for label smoothing, 0 means no label smoothing')
parser.add_argument('--report-accuracy', action='store_true',
help='report accuracy metric')
parser.add_argument('--ignore-prefix-size', default=0, type=int,
help='Ignore first N tokens')
# fmt: on
[docs] def forward(self, model, sample, reduce=True):
"""Compute the loss for the given sample.
Returns a tuple with three elements:
1) the loss
2) the sample size, which is used as the denominator for the gradient
3) logging outputs to display while training
"""
net_output = model(**sample["net_input"])
loss, nll_loss = self.compute_loss(model, net_output, sample, reduce=reduce)
sample_size = (
sample["target"].size(0) if self.sentence_avg else sample["ntokens"]
)
logging_output = {
"loss": loss.data,
"nll_loss": nll_loss.data,
"ntokens": sample["ntokens"],
"nsentences": sample["target"].size(0),
"sample_size": sample_size,
}
if self.report_accuracy:
n_correct, total = self.compute_accuracy(model, net_output, sample)
logging_output["n_correct"] = utils.item(n_correct.data)
logging_output["total"] = utils.item(total.data)
return loss, sample_size, logging_output
[docs] def get_lprobs_and_target(self, model, net_output, sample):
lprobs = model.get_normalized_probs(net_output, log_probs=True)
target = model.get_targets(sample, net_output)
if self.ignore_prefix_size > 0:
if getattr(lprobs, "batch_first", False):
lprobs = lprobs[:, self.ignore_prefix_size :, :].contiguous()
target = target[:, self.ignore_prefix_size :].contiguous()
else:
lprobs = lprobs[self.ignore_prefix_size :, :, :].contiguous()
target = target[self.ignore_prefix_size :, :].contiguous()
return lprobs.view(-1, lprobs.size(-1)), target.view(-1)
[docs] def compute_loss(self, model, net_output, sample, reduce=True):
lprobs, target = self.get_lprobs_and_target(model, net_output, sample)
loss, nll_loss = label_smoothed_nll_loss(
lprobs,
target,
self.eps,
ignore_index=self.padding_idx,
reduce=reduce,
)
return loss, nll_loss
[docs] def compute_accuracy(self, model, net_output, sample):
lprobs, target = self.get_lprobs_and_target(model, net_output, sample)
mask = target.ne(self.padding_idx)
n_correct = torch.sum(
lprobs.argmax(1).masked_select(mask).eq(target.masked_select(mask))
)
total = torch.sum(mask)
return n_correct, total
[docs] @classmethod
def reduce_metrics(cls, logging_outputs) -> None:
"""Aggregate logging outputs from data parallel training."""
loss_sum = sum(log.get("loss", 0) for log in logging_outputs)
nll_loss_sum = sum(log.get("nll_loss", 0) for log in logging_outputs)
ntokens = sum(log.get("ntokens", 0) for log in logging_outputs)
sample_size = sum(log.get("sample_size", 0) for log in logging_outputs)
metrics.log_scalar(
"loss", loss_sum / sample_size / math.log(2), sample_size, round=3
)
metrics.log_scalar(
"nll_loss", nll_loss_sum / ntokens / math.log(2), ntokens, round=3
)
metrics.log_derived(
"ppl", lambda meters: utils.get_perplexity(meters["nll_loss"].avg)
)
total = utils.item(sum(log.get("total", 0) for log in logging_outputs))
if total > 0:
metrics.log_scalar("total", total)
n_correct = utils.item(
sum(log.get("n_correct", 0) for log in logging_outputs)
)
metrics.log_scalar("n_correct", n_correct)
metrics.log_derived(
"accuracy",
lambda meters: round(
meters["n_correct"].sum * 100.0 / meters["total"].sum, 3
)
if meters["total"].sum > 0
else float("nan"),
)
[docs] @staticmethod
def logging_outputs_can_be_summed() -> bool:
"""
Whether the logging outputs returned by `forward` can be summed
across workers prior to calling `reduce_metrics`. Setting this
to True will improves distributed training speed.
"""
return True