    def build_criterion(cls, cfg: FairseqDataclass, task):
        """Construct a criterion from command-line args."""
        # arguments in the __init__.
        init_args = {}
        for p in inspect.signature(cls).parameters.values():
            if (
                p.kind == p.POSITIONAL_ONLY
                or p.kind == p.VAR_POSITIONAL
                or p.kind == p.VAR_KEYWORD
            ):
                # we haven't implemented inference for these argument types,
                # but PRs welcome :)
                raise NotImplementedError("{} not supported".format(p.kind))

            assert p.kind in {p.POSITIONAL_OR_KEYWORD, p.KEYWORD_ONLY}

            if p.name == "task":
                init_args["task"] = task
            elif p.name == "cfg":
                init_args["cfg"] = cfg
            elif hasattr(cfg, p.name):
                init_args[p.name] = getattr(cfg, p.name)
            elif p.default != p.empty:
                pass  # we'll use the default value
            else:
                raise NotImplementedError(
                    "Unable to infer Criterion arguments, please implement "
                    "{}.build_criterion".format(cls.__name__)
                )
        return cls(**init_args)
Construct a criterion from command-line args.
fairseq/criterions/fairseq_criterion.py