Encode a single token byte-wise into integer ids.

def _byte_encode(self, token):
    """Encode a single token byte-wise into integer ids."""
    # Vocab ids for all bytes follow ids for the subwords
    offset = len(self._subwords)
    if token == "_":
      return [len(self._subwords) + ord(" ")]
    return [i + offset for i in list(bytearray(tf.compat.as_bytes(token)))]