code: def start_index(self) -> Union[int, None]:
      if not self.uri.exists():
          return None
      with self.uri.open("rb") as fp:
          index = int(np.frombuffer(fp.read(4), dtype="<f")[0])
      return index
  
imports: import struct
  from pathlib import Path
  from typing import Iterable, Union, Dict, Mapping, Tuple, List
  import numpy
  import pandas
  from qlib.utils.time import Freq
  from qlib.utils.resam import resam_calendar
  from qlib.config import C
  from qlib.data.cache import H
  from qlib.log import get_module_logger
  from qlib.data.storage import CalendarStorage, InstrumentStorage, FeatureStorage, CalVT, InstKT, InstVT
label_desc: Use image node_id 5 for calling the FileFeatureStorage obj's underlying member method code with example usage: obj.start_index() and returns: index, None