class_docstr: FVQA Teacher, which loads the json VQA data and implements its own `act` method for
  interacting with student agent.
  
  Use "fvqa:split:X" to choose between splits 0-4 (inclusive), or just "fvqa" to use
  the default split (0).
code: def num_examples(self):
      return self.len
  
imports: from parlai.core.image_featurizers import ImageLoader
  from parlai.core.metrics import TeacherMetrics
  from parlai.core.teachers import Teacher
  from parlai.utils.io import PathManager
  from .build import build
  import json
  import random
  import os
label_desc: Use image node_id 2 for calling the SplitTeacher obj's underlying member method code with example usage: obj.num_examples() and returns: self