docstr: Add criterion-specific arguments to the parser.
code: def add_args(cls, 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",
      )
  
file_path: translate/pytorch_translate/weighted_criterions.py
imports: from fairseq import utils
  from fairseq.criterions import LegacyFairseqCriterion, register_criterion
  from fairseq.criterions.label_smoothed_cross_entropy import LabelSmoothedCrossEntropyCriterion
label_desc: Use image node_id 2 for calling the WeightedLabelSmoothedCrossEntropyCriterion obj's underlying member method code with example usage: obj.add_args(cls, parser) without return types