parameters: (self, word)
return_statement: return self.word2id.get(word, self.unk_id)
docstring: Retrieve word's index. Return the index for the unk
          token if the word is out of vocabulary.
          @param word (str): word to look up.
          @returns index (int): index of word
docstring_summary: Retrieve word's index. Return the index for the unk
          token if the word is out of vocabulary.
docstring_tokens: Retrieve word s index . Return the index for the unk token if the word is out of vocabulary .
function: def __getitem__(self, word):
          """ Retrieve word's index. Return the index for the unk
          token if the word is out of vocabulary.
          @param word (str): word to look up.
          @returns index (int): index of word 
          """
          return self.word2id.get(word, self.unk_id)
function_tokens: def __getitem__ ( self , word ) : return self . word2id . get ( word , self . unk_id )