src/transformers/convert_slow_tokenizer.py/T5Converter/vocab
class T5Converter:    def vocab(self, proto):
        num_extra_ids = self.original_tokenizer._extra_ids
        vocab = [(piece.piece, piece.score) for piece in proto.pieces]
        vocab += [(f"<extra_id_{i}>", 0.0) for i in range(num_extra_ids - 1, -1, -1)]
        return vocab