src/transformers/models/bert_generation/tokenization_bert_generation.py/BertGenerationTokenizer/_convert_id_to_token
class BertGenerationTokenizer:    def _convert_id_to_token(self, index):
        """Converts an index (integer) in a token (str) using the vocab."""
        token = self.sp_model.IdToPiece(index)
        return token