    def _map_index_to_dataset(self, key: int, index: int):
        """
        Different underlying datasets have different lengths. In order to ensure
        we are not accessing an index outside the range of the current dataset
        size, we wrap around. This function should be called after we have
        created an ordering for this and all underlying datasets.
        """
        assert (
            self._ordered_indices is not None
        ), "Must call MultiCorpusSampledDataset.ordered_indices() first"
        mapped_index = index % len(self.datasets[key])
        return self._ordered_indices[key][mapped_index]
Different underlying datasets have different lengths. In order to ensure
we are not accessing an index outside the range of the current dataset
size, we wrap around. This function should be called after we have
created an ordering for this and all underlying datasets.
fairseq/data/multi_corpus_sampled_dataset.py