    def _all_gather_list_sync(
        self,
        logging_outputs: List[Dict[str, Any]],
        *extra_stats_to_sum,
        ignore=False,
    ):
        """
        Sync logging outputs across workers. all_gather_list_sync is
        suitable when logging outputs are complex types.
        """
        if self.tpu:
            raise NotImplementedError
        if ignore:
            logging_outputs = []
        results = list(
            zip(
                *distributed_utils.all_gather_list(
                    [logging_outputs] + list(extra_stats_to_sum),
                    max_size=getattr(self.cfg.common, "all_gather_list_size", 16384),
                    group=self.data_parallel_process_group,
                )
            )
        )
        logging_outputs, extra_stats_to_sum = results[0], results[1:]
        logging_outputs = list(chain.from_iterable(logging_outputs))
        extra_stats_to_sum = [sum(s) for s in extra_stats_to_sum]
        return logging_outputs, extra_stats_to_sum
Sync logging outputs across workers. all_gather_list_sync is
suitable when logging outputs are complex types.
fairseq/trainer.py