class_docstr: A mapping from indices to vocab elements.
class_docstr_tok: ["A","mapping","from","indices","to","vocab","elements","."]
code: def get_mask_index(self, value=None):
      if value is None:
          return self.idx[self.mask_token]
      else:
          return self.idx.get(self.mask_token, value)
  
imports: import itertools
  from collections import Counter
  from typing import Dict, List, Optional, Tuple
  import torch
  from pytext.common.constants import SpecialTokens, Token
  from pytext.utils import cuda, precision
label_desc: Use image node_id 7 for calling the Vocabulary obj's underlying member method code with example usage: obj.get_mask_index(value) and returns: self, self