    def build_generator(
        self,
        models,
        args,
        seq_gen_cls=None,
        extra_gen_cls_kwargs=None,
        prefix_allowed_tokens_fn=None,
    ):
        """
        Build a :class:`~fairseq.SequenceGenerator` instance for this
        task.

        Args:
            models (List[~fairseq.models.FairseqModel]): ensemble of models
            args (fairseq.dataclass.configs.GenerationConfig):
                configuration object (dataclass) for generation
            extra_gen_cls_kwargs (Dict[str, Any]): extra options to pass
                through to SequenceGenerator
            prefix_allowed_tokens_fn (Callable[[int, torch.Tensor], List[int]]):
                If provided, this function constrains the beam search to
                allowed tokens only at each step. The provided function
                should take 2 arguments: the batch ID (`batch_id: int`)
                and a unidimensional tensor of token ids (`inputs_ids:
                torch.Tensor`). It has to return a `List[int]` with the
                allowed tokens for the next generation step conditioned
                on the previously generated tokens (`inputs_ids`) and
                the batch ID (`batch_id`). This argument is useful for
                constrained generation conditioned on the prefix, as
                described in "Autoregressive Entity Retrieval"
                (https://arxiv.org/abs/2010.00904) and
                https://github.com/facebookresearch/GENRE.
        """
        if getattr(args, "score_reference", False):
            from fairseq.sequence_scorer import SequenceScorer

            return SequenceScorer(
                self.target_dictionary,
                compute_alignment=getattr(args, "print_alignment", False),
            )

        from fairseq.sequence_generator import (
            SequenceGenerator,
            SequenceGeneratorWithAlignment,
        )

        # Choose search strategy. Defaults to Beam Search.
        sampling = getattr(args, "sampling", False)
        sampling_topk = getattr(args, "sampling_topk", -1)
        sampling_topp = getattr(args, "sampling_topp", -1.0)
        diverse_beam_groups = getattr(args, "diverse_beam_groups", -1)
        diverse_beam_strength = getattr(args, "diverse_beam_strength", 0.5)
        match_source_len = getattr(args, "match_source_len", False)
        diversity_rate = getattr(args, "diversity_rate", -1)
        constrained = getattr(args, "constraints", False)
        if prefix_allowed_tokens_fn is None:
            prefix_allowed_tokens_fn = getattr(args, "prefix_allowed_tokens_fn", None)
        if (
            sum(
                int(cond)
                for cond in [
                    sampling,
                    diverse_beam_groups > 0,
                    match_source_len,
                    diversity_rate > 0,
                ]
            )
            > 1
        ):
            raise ValueError("Provided Search parameters are mutually exclusive.")
        assert sampling_topk < 0 or sampling, "--sampling-topk requires --sampling"
        assert sampling_topp < 0 or sampling, "--sampling-topp requires --sampling"

        if sampling:
            search_strategy = search.Sampling(
                self.target_dictionary, sampling_topk, sampling_topp
            )
        elif diverse_beam_groups > 0:
            search_strategy = search.DiverseBeamSearch(
                self.target_dictionary, diverse_beam_groups, diverse_beam_strength
            )
        elif match_source_len:
            # this is useful for tagging applications where the output
            # length should match the input length, so we hardcode the
            # length constraints for simplicity
            search_strategy = search.LengthConstrainedBeamSearch(
                self.target_dictionary,
                min_len_a=1,
                min_len_b=0,
                max_len_a=1,
                max_len_b=0,
            )
        elif diversity_rate > -1:
            search_strategy = search.DiverseSiblingsSearch(
                self.target_dictionary, diversity_rate
            )
        elif constrained:
            search_strategy = search.LexicallyConstrainedBeamSearch(
                self.target_dictionary, args.constraints
            )
        elif prefix_allowed_tokens_fn:
            search_strategy = search.PrefixConstrainedBeamSearch(
                self.target_dictionary, prefix_allowed_tokens_fn
            )
        else:
            search_strategy = search.BeamSearch(self.target_dictionary)

        extra_gen_cls_kwargs = extra_gen_cls_kwargs or {}
        if seq_gen_cls is None:
            if getattr(args, "print_alignment", False):
                seq_gen_cls = SequenceGeneratorWithAlignment
                extra_gen_cls_kwargs["print_alignment"] = args.print_alignment
            else:
                seq_gen_cls = SequenceGenerator

        return seq_gen_cls(
            models,
            self.target_dictionary,
            beam_size=getattr(args, "beam", 5),
            max_len_a=getattr(args, "max_len_a", 0),
            max_len_b=getattr(args, "max_len_b", 200),
            min_len=getattr(args, "min_len", 1),
            normalize_scores=(not getattr(args, "unnormalized", False)),
            len_penalty=getattr(args, "lenpen", 1),
            unk_penalty=getattr(args, "unkpen", 0),
            temperature=getattr(args, "temperature", 1.0),
            match_source_len=getattr(args, "match_source_len", False),
            no_repeat_ngram_size=getattr(args, "no_repeat_ngram_size", 0),
            search_strategy=search_strategy,
            **extra_gen_cls_kwargs,
        )
Build a :class:`~fairseq.SequenceGenerator` instance for this
task.

Args:
models (List[~fairseq.models.FairseqModel]): ensemble of models
args (fairseq.dataclass.configs.GenerationConfig):
configuration object (dataclass) for generation
extra_gen_cls_kwargs (Dict[str, Any]): extra options to pass
through to SequenceGenerator
prefix_allowed_tokens_fn (Callable[[int, torch.Tensor], List[int]]):
If provided, this function constrains the beam search to
allowed tokens only at each step. The provided function
should take 2 arguments: the batch ID (`batch_id: int`)
and a unidimensional tensor of token ids (`inputs_ids:
torch.Tensor`). It has to return a `List[int]` with the
allowed tokens for the next generation step conditioned
on the previously generated tokens (`inputs_ids`) and
the batch ID (`batch_id`). This argument is useful for
constrained generation conditioned on the prefix, as
described in "Autoregressive Entity Retrieval"
(https://arxiv.org/abs/2010.00904) and
https://github.com/facebookresearch/GENRE.
fairseq/tasks/fairseq_task.py