    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 (
            self.src_sizes[index],
            self.tgt_sizes[index] if self.tgt_sizes is not None else 0,
        )
Return an example's size as a float or tuple. This value is used when
filtering a dataset with ``--max-positions``.
fairseq/data/language_pair_dataset.py