Indexes unknown and reserved tokens.

def _index_unknown_and_reserved_tokens(self, unknown_token, reserved_tokens):
        """Indexes unknown and reserved tokens."""

        self._unknown_token = unknown_token
        # Thus, constants.UNKNOWN_IDX must be 0.
        self._idx_to_token = [unknown_token]

        if reserved_tokens is None:
            self._reserved_tokens = None
        else:
            self._reserved_tokens = reserved_tokens[:]
            self._idx_to_token.extend(reserved_tokens)

        self._token_to_idx = {token: idx for idx, token in enumerate(self._idx_to_token)}