    def __getitem__(self, index: int):
        """
        Get the item associated with index from each underlying dataset.
        Since index is in the range of [0, TotalNumInstances], we need to
        map the index to the dataset before retrieving the item.
        """
        return OrderedDict(
            [
                (key, dataset[self._map_index_to_dataset(key, index)])
                for key, dataset in self.datasets.items()
            ]
        )
Get the item associated with index from each underlying dataset.
Since index is in the range of [0, TotalNumInstances], we need to
map the index to the dataset before retrieving the item.
fairseq/data/multi_corpus_sampled_dataset.py