Converts a subword integer ID to a subword string.

def _id_to_subword(self, subword_id):
    """Converts a subword integer ID to a subword string."""
    if subword_id < 0 or subword_id >= (self.vocab_size - 1):
      raise ValueError("Received id %d which is invalid. Ids must be within "
                       "[0, %d)." % (subword_id + 1, self.vocab_size))

    if 0 <= subword_id < len(self._subwords):
      # Subword
      return self._subwords[subword_id]
    else:
      # Byte
      offset = len(self._subwords)
      subword_id -= offset
      bytestr = bytes(bytearray([subword_id]))
      return bytestr