    def ordered_indices(self):
        """
        Ordered indices for batching. Here we call the underlying
        dataset's ordered_indices() so that we get the same random ordering
        as we would have from using the underlying dataset directly.
        """
        if self._ordered_indices is None:
            self._ordered_indices = OrderedDict(
                [
                    (key, dataset.ordered_indices())
                    for key, dataset in self.datasets.items()
                ]
            )
        return np.arange(len(self))
Ordered indices for batching. Here we call the underlying
dataset's ordered_indices() so that we get the same random ordering
as we would have from using the underlying dataset directly.
fairseq/data/multi_corpus_sampled_dataset.py