class_docstr: This version of SQuAD inherits from the core Dialog Teacher, which just requires it
  to define an iterator over its data `setup_data` in order to inherit basic metrics,
  a default `act` function.
  
  Note: This teacher omits the context paragraph
code: def __init__(self, opt, shared=None):
      self.datatype = opt["datatype"]
      build(opt)
      if opt["datatype"].startswith("train"):
          suffix = "train"
      else:
          suffix = "dev"
      opt["datafile"] = os.path.join(
          opt["datapath"], "SQuAD2", suffix + "-v2.0.json"
      )
      self.id = "squad2"
      super().__init__(opt, shared)
  
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 2 to create a new OpenSquadTeacher object from inherited base classes: DialogTeacher with example: obj = OpenSquadTeacher(opt, shared)