index-file

在retrieval searching的代码中显示需要index_file，请问index_file是什么
usage: retrieval.py [-h] [--model_path MODEL_PATH] [--pool_type POOL_TYPE] [--max_sequence_length MAX_SEQUENCE_LENGTH] [--index_file INDEX_FILE] [--topk TOPK] [--input_file INPUT_FILE] [--output_file OUTPUT_FILE]

optional arguments:
-h, --help show this help message and exit
--model_path MODEL_PATH
--pool_type POOL_TYPE
pool type of the final text repsesentation
--max_sequence_length MAX_SEQUENCE_LENGTH
use pb model or tf checkpoint
--index_file INDEX_FILE
--topk TOPK
--input_file INPUT_FILE
--output_file OUTPUT_FILE