    def size(self, index):
        """Return an example's size as a float or tuple. This value is used when
        filtering a dataset with ``--max-positions``."""
        return {
            key: dataset.size(self._map_index(key, index))
            for key, dataset in self.datasets.items()
        }
Return an example's size as a float or tuple. This value is used when
filtering a dataset with ``--max-positions``.
fairseq/data/round_robin_zip_datasets.py