class_docstr: A base class for common vocabulary operations. Each subclass should at least 
  implement its own build_vocab() function.
code: def map(self, units):
      return [self.unit2id(x) for x in units]
  
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 8 for calling the BaseVocab obj's underlying member method code with example usage: obj.map(units) and returns: unknown