# 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 dataclasses import dataclass
import torch.nn.functional as F
from fairseq import metrics, utils
from fairseq.criterions import FairseqCriterion, register_criterion
from fairseq.dataclass import FairseqDataclass
from fairseq.dataclass.constants import DDP_BACKEND_CHOICES
from omegaconf import II
@dataclass
class AdaptiveLossConfig(FairseqDataclass):
sentence_avg: bool = II("optimization.sentence_avg")
ddp_backend: DDP_BACKEND_CHOICES = II("distributed_training.ddp_backend")
[docs]@register_criterion("adaptive_loss", dataclass=AdaptiveLossConfig)
class AdaptiveLoss(FairseqCriterion):
"""This is an implementation of the loss function accompanying the adaptive softmax approximation for
graphical processing units (GPU), described in the paper "Efficient softmax approximation for GPUs"
(http://arxiv.org/abs/1609.04309)."""
def __init__(self, task, sentence_avg):
super().__init__(task)
self.sentence_avg = sentence_avg
[docs] @classmethod
def build_criterion(cls, cfg: AdaptiveLossConfig, task):
if cfg.ddp_backend in {"c10d", "pytorch_ddp"}:
raise Exception(
"AdaptiveLoss is not compatible with the PyTorch "
"version of DistributedDataParallel. Please use "
"`--ddp-backend=legacy_ddp` instead."
)
return cls(task, cfg.sentence_avg)
[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
"""
assert (
hasattr(model.decoder, "adaptive_softmax")
and model.decoder.adaptive_softmax is not None
)
adaptive_softmax = model.decoder.adaptive_softmax
net_output = model(**sample["net_input"])
orig_target = model.get_targets(sample, net_output)
nsentences = orig_target.size(0)
orig_target = orig_target.view(-1)
bsz = orig_target.size(0)
logits, target = adaptive_softmax(net_output[0], orig_target)
assert len(target) == len(logits)
loss = net_output[0].new(1 if reduce else bsz).zero_()
for i in range(len(target)):
if target[i] is not None:
assert target[i].min() >= 0 and target[i].max() <= logits[i].size(1)
loss += F.cross_entropy(
logits[i],
target[i],
ignore_index=self.padding_idx,
reduction="sum" if reduce else "none",
)
orig = utils.strip_pad(orig_target, self.padding_idx)
ntokens = orig.numel()
sample_size = sample["target"].size(0) if self.sentence_avg else ntokens
logging_output = {
"loss": loss.data,
"ntokens": ntokens,
"nsentences": nsentences,
"sample_size": sample_size,
}
return loss, sample_size, logging_output
[docs] @staticmethod
def reduce_metrics(logging_outputs) -> None:
"""Aggregate logging outputs from data parallel training."""
loss_sum = utils.item(sum(log.get("loss", 0) for log in logging_outputs))
ntokens = utils.item(sum(log.get("ntokens", 0) for log in logging_outputs))
sample_size = utils.item(
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
)
if sample_size != ntokens:
metrics.log_scalar(
"nll_loss", loss_sum / ntokens / math.log(2), ntokens, round=3
)
metrics.log_derived(
"ppl", lambda meters: utils.get_perplexity(meters["nll_loss"].avg)
)
else:
metrics.log_derived(
"ppl", lambda meters: utils.get_perplexity(meters["loss"].avg)
)
[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