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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 math
import torch
import torch.nn.functional as F
from fairseq import metrics, utils
from fairseq.criterions import FairseqCriterion, register_criterion
from torch import Tensor
@register_criterion("nat_loss")
class LabelSmoothedDualImitationCriterion(FairseqCriterion):
def __init__(self, task, label_smoothing):
super().__init__(task)
self.label_smoothing = label_smoothing
@staticmethod
def add_args(parser):
"""Add criterion-specific arguments to the parser."""
parser.add_argument(
"--label-smoothing",
default=0.0,
type=float,
metavar="D",
help="epsilon for label smoothing, 0 means no label smoothing",
)
def _compute_loss(
self, outputs, targets, masks=None, label_smoothing=0.0, name="loss", factor=1.0
):
"""
outputs: batch x len x d_model
targets: batch x len
masks: batch x len
policy_logprob: if there is some policy
depends on the likelihood score as rewards.
"""
def mean_ds(x: Tensor, dim=None) -> Tensor:
return (
x.float().mean().type_as(x)
if dim is None
else x.float().mean(dim).type_as(x)
)
if masks is not None:
outputs, targets = outputs[masks], targets[masks]
if masks is not None and not masks.any():
nll_loss = torch.tensor(0)
loss = nll_loss
else:
logits = F.log_softmax(outputs, dim=-1)
if targets.dim() == 1:
losses = F.nll_loss(logits, targets.to(logits.device), reduction="none")
else: # soft-labels
losses = F.kl_div(logits, targets.to(logits.device), reduction="none")
losses = losses.sum(-1)
nll_loss = mean_ds(losses)
if label_smoothing > 0:
loss = (
nll_loss * (1 - label_smoothing) - mean_ds(logits) * label_smoothing
)
else:
loss = nll_loss
loss = loss * factor
return {"name": name, "loss": loss, "nll_loss": nll_loss, "factor": factor}
def _custom_loss(self, loss, name="loss", factor=1.0):
return {"name": name, "loss": loss, "factor": factor}
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
"""
nsentences, ntokens = sample["nsentences"], sample["ntokens"]
# B x T
src_tokens, src_lengths = (
sample["net_input"]["src_tokens"],
sample["net_input"]["src_lengths"],
)
tgt_tokens, prev_output_tokens = sample["target"], sample["prev_target"]
outputs = model(src_tokens, src_lengths, prev_output_tokens, tgt_tokens)
losses, nll_loss = [], []
for obj in outputs:
if outputs[obj].get("loss", None) is None:
_losses = self._compute_loss(
outputs[obj].get("out"),
outputs[obj].get("tgt"),
outputs[obj].get("mask", None),
outputs[obj].get("ls", 0.0),
name=obj + "-loss",
factor=outputs[obj].get("factor", 1.0),
)
else:
_losses = self._custom_loss(
outputs[obj].get("loss"),
name=obj + "-loss",
factor=outputs[obj].get("factor", 1.0),
)
losses += [_losses]
if outputs[obj].get("nll_loss", False):
nll_loss += [_losses.get("nll_loss", 0.0)]
loss = sum(l["loss"] for l in losses)
nll_loss = sum(l for l in nll_loss) if len(nll_loss) > 0 else loss.new_tensor(0)
# NOTE:
# we don't need to use sample_size as denominator for the gradient
# here sample_size is just used for logging
sample_size = 1
logging_output = {
"loss": loss.data,
"nll_loss": nll_loss.data,
"ntokens": ntokens,
"nsentences": nsentences,
"sample_size": sample_size,
}
for l in losses:
logging_output[l["name"]] = (
utils.item(l["loss"].data / l["factor"])
if reduce
else l[["loss"]].data / l["factor"]
)
return loss, sample_size, logging_output
@staticmethod
def reduce_metrics(logging_outputs) -> None:
"""Aggregate logging outputs from data parallel training."""
sample_size = utils.item(
sum(log.get("sample_size", 0) for log in logging_outputs)
)
loss = utils.item(sum(log.get("loss", 0) for log in logging_outputs))
nll_loss = utils.item(sum(log.get("nll_loss", 0) for log in logging_outputs))
metrics.log_scalar(
"loss", loss / sample_size / math.log(2), sample_size, round=3
)
metrics.log_scalar(
"nll_loss", nll_loss / sample_size / math.log(2), sample_size, round=3
)
metrics.log_derived(
"ppl", lambda meters: utils.get_perplexity(meters["loss"].avg)
)
for key in logging_outputs[0]:
if key[-5:] == "-loss":
val = sum(log.get(key, 0) for log in logging_outputs)
metrics.log_scalar(
key[:-5],
val / sample_size / math.log(2) if sample_size > 0 else 0.0,
sample_size,
round=3,
)
@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
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