examples/legacy/seq2seq/utils.py/Seq2SeqDataset/__getitem__
class Seq2SeqDataset:    def __getitem__(self, index) -> Dict[str, str]:
        index = index + 1  # linecache starts at 1
        source_line = self.prefix + linecache.getline(str(self.src_file), index).rstrip("\n")
        tgt_line = linecache.getline(str(self.tgt_file), index).rstrip("\n")
        assert source_line, f"empty source line for index {index}"
        assert tgt_line, f"empty tgt line for index {index}"
        return {"tgt_texts": tgt_line, "src_texts": source_line, "id": index - 1}