class_docstr: Random mask tokens from src sentence.
docstr: Get special vocabs added by prefix transform.
docstr_tok: ["Get","special","vocabs","added","by","prefix","transform","."]
code: def get_specials(cls, opts):
      """Get special vocabs added by prefix transform."""
      return ({cls.MASK_TOK}, set())
  
imports: import random
  import numpy
  from onmt.constants import DefaultTokens
  from onmt.transforms import register_transform
  from .transform import Transform, ObservableStats
label_desc: Use image node_id 4 for calling the TokenMaskTransform obj's underlying member method code with example usage: obj.get_specials(cls, opts) and returns: unknown,  set