    def batch_instances(
        self, epoch_instances: Dict[str, Iterable[Instance]]
    ) -> Iterable[List[Instance]]:
        """
        Given a dictionary of `Iterable[Instance]` for each dataset, combines them into an
        `Iterable` of batches of instances.
        """
        raise NotImplementedError
Given a dictionary of `Iterable[Instance]` for each dataset, combines them into an
`Iterable` of batches of instances.
allennlp/data/data_loaders/multitask_scheduler.py