class_docstr: Dictionary for Masked Language Modelling tasks. This extends Dictionary by
  adding the mask symbol.
docstr: Helper to get index of mask symbol
docstr_tok: ["Helper","to","get","index","of","mask","symbol"]
code: def mask(self):
      """Helper to get index of mask symbol"""
      return self.mask_index
  
imports: from typing import Dict, List, Set
  from pytorch_translate import vocab_constants
  from pytorch_translate.data.dictionary import Dictionary
label_desc: Use image node_id 2 for calling the MaskedLMDictionary obj's underlying member method code with example usage: obj.mask() and returns: self