    def encode(self, iter: tp.List[str]) -> bytes:
        """
        encode a list of tokens a return bytes. We use bitpadding to make sure the encoded bits fit in bytes.
        """
        a = bitarray()
        for token in iter:
            code = self.get_code(token)
            if code is None:
                if self.unk_word is None:
                    raise Exception(f"unknown token {token} cannot be encoded.")
                else:
                    token = self.unk_word
            a = a + self.get_code(token)
        return self._pad(a).tobytes()
encode a list of tokens a return bytes. We use bitpadding to make sure the encoded bits fit in bytes.
fairseq/data/huffman/huffman_coder.py