Converts a list of tokens to a list of subtoken.

        Args:
          tokens: a list of strings.
        Returns:
          a list of integers in the range [0, vocab_size)

def _tokens_to_subtoken(self, tokens):
        """ Converts a list of tokens to a list of subtoken.

        Args:
          tokens: a list of strings.
        Returns:
          a list of integers in the range [0, vocab_size)
        """
        ret = []
        for token in tokens:
            ret.extend(
                self._escaped_token_to_subtoken_strings(_escape_token(token, self._alphabet)))
        return ret