code: def add_common_cmdline_args(parser):
      agent = parser.add_argument_group("Squad2 teacher arguments")
      agent.add_argument(
          "--impossible-answer-string",
          type=str,
          default="",
          help='Set the label for impossible answers; defaults to an empty string, but one might try something like "I do not know"',
      )
  
imports: from typing import Optional
  from parlai.core.params import ParlaiParser
  from parlai.core.opt import Opt
  from parlai.core.teachers import FixedDialogTeacher, DialogTeacher
  from parlai.utils.io import PathManager
  from .build import build
  import json
  import os
label_desc: Use image node_id 1 for calling a global function with example usage: add_common_cmdline_args(parser) without return types