class_docstr: Vocabulary class that loads vocabulary from file, and maintains mapping
  tables between token strings and indexes.
  
  Each line of the vocab file should contains one vocabulary token, e.g.,::
  
      vocab_token_1
      vocab token 2
      vocab       token | 3 .
      ...
  
  Args:
      filename (str): Path to the vocabulary file where each line contains
          one token.
      bos_token (str): A special token that will be added to the beginning of
          sequences.
      eos_token (str): A special token that will be added to the end of
          sequences.
      unk_token (str): A special token that will replace all unknown tokens
          (tokens not included in the vocabulary).
      pad_token (str): A special token that is used to do padding.
class_docstr_tok: ["Vocabulary","class","that","loads","vocabulary","from","file",",","and","maintains","mapping","tables","between","token","strings","and","indexes",".","Each","line","of","the","vocab","file","should","contains","one","vocabulary","token",",","e.g.",",",":",":","vocab_token_1","vocab","token","2","vocab","token","|","3",".","...","Args",":","filename","(","str",")",":","Path","to","the","vocabulary","file","where","each","line","contains","one","token",".","bos_token","(","str",")",":","A","special","token","that","will","be","added","to","the","beginning","of","sequences",".","eos_token","(","str",")",":","A","special","token","that","will","be","added","to","the","end","of","sequences",".","unk_token","(","str",")",":","A","special","token","that","will","replace","all","unknown","tokens","(","tokens","not","included","in","the","vocabulary",")",".","pad_token","(","str",")",":","A","special","token","that","is","used","to","do","padding","."]
docstr: A string of the special token indicating unknown token.
          
docstr_tok: ["A","string","of","the","special","token","indicating","unknown","token","."]
code: def unk_token(self):
      """A string of the special token indicating unknown token."""
      return self._unk_token
  
imports: import warnings
  from collections import defaultdict
  import tensorflow
  from tensorflow import gfile
  import numpy
  from texar.tf.utils.utils import dict_lookup
label_desc: Use image node_id 16 for calling the Vocab obj's underlying member method code with example usage: obj.unk_token() and returns: self