class_docstr: A convenient vocab container that can store multiple BaseVocab instances, and support 
  safe serialization of all instances via state dicts. Each subclass of this base class 
  should implement the load_state_dict() function to specify how a saved state dict 
  should be loaded back.
code: def __contains__(self, key):
      return key in self._vocabs
  
imports: from copy import copy
  from collections import Counter, OrderedDict
  from collections.abc import Iterable
  import os
  import pickle
label_desc: Use image node_id 5 for calling the BaseMultiVocab obj's underlying member method code with example usage: obj.__contains__(key) and returns: unknown