class_docstr: Teacher that corresponds to the default DSTC7 ubuntu track 1.
  
  The data hasn't been augmented by using the multi-turn utterances.
code: def num_examples(self):
      return len(self.data)
  
imports: from parlai.core.teachers import FixedDialogTeacher
  from parlai.utils.io import PathManager
  from .build import build
  import json
  import os
  import random
label_desc: Use image node_id 5 for calling the DSTC7Teacher obj's underlying member method code with example usage: obj.num_examples() and returns: len