docstr: get FeatureStorage start index
  
  Notes
  -----
  If the data(storage) does not exist, return None
docstr_tok: ["get","FeatureStorage","start","index","Notes","--","--","-","If","the","data","(","storage",")","does","not","exist",",","return","None"]
code: def start_index(self) -> Union[int, None]:
      """get FeatureStorage start index
  
      Notes
      -----
      If the data(storage) does not exist, return None
      """
      raise NotImplementedError(
          "Subclass of FeatureStorage must implement `start_index` method"
      )
  
imports: import re
  from typing import Iterable, overload, Tuple, List, Text, Union, Dict
  import numpy
  import pandas
  from qlib.log import get_module_logger
label_desc: Use image node_id 3 for calling the FeatureStorage obj's underlying member method code with example usage: obj.start_index() without return types