code: def _tokens_to_subtoken_ids(self, tokens):
      
      ret = []
      for token in tokens:
        ret.extend(self._token_to_subtoken_ids(token))
      return ret
docs: Converts a list of tokens to a list of subtoken ids.
  
      Args:
        tokens: a list of strings.
      Returns:
        a list of integers in the range [0, vocab_size)