Indexes unknown and reserved tokens.

def _index_special_tokens(self, unknown_token, special_tokens):
        """Indexes unknown and reserved tokens."""
        self._idx_to_token = [unknown_token] if unknown_token else []

        if not special_tokens:
            self._reserved_tokens = None
        else:
            self._reserved_tokens = special_tokens[:]
            self._idx_to_token.extend(special_tokens)

        if unknown_token:
            self._token_to_idx = DefaultLookupDict(C.UNK_IDX)
        else:
            self._token_to_idx = {}
        self._token_to_idx.update((token, idx) for idx, token in enumerate(self._idx_to_token))