docstr: Returns
  -------
      index start date
docstr_tok: ["Returns","--","--","--","-","index","start","date"]
code: def bench_start_date(self) -> pd.Timestamp:
      """
      Returns
      -------
          index start date
      """
      raise NotImplementedError("rewrite bench_start_date")
  
file_path: qlib/scripts/data_collector/index.py
imports: import sys
  import abc
  from pathlib import Path
  from typing import List
  import pandas
  from tqdm import tqdm
  from loguru import logger
  from data_collector.utils import get_trading_date_by_shift
label_desc: Use image node_id 2 for calling the IndexBase obj's underlying member method code with example usage: obj.bench_start_date() without return types