Transactionally commit the changes accumulated.

        Returns:
            List[google.cloud.proto.firestore.v1beta1.\
                write_pb2.WriteResult, ...]: The write results corresponding
            to the changes committed, returned in the same order as the
            changes were applied to this transaction. A write result contains
            an ``update_time`` field.

        Raises:
            ValueError: If no transaction is in progress.

def _commit(self):
        """Transactionally commit the changes accumulated.

        Returns:
            List[google.cloud.proto.firestore.v1beta1.\
                write_pb2.WriteResult, ...]: The write results corresponding
            to the changes committed, returned in the same order as the
            changes were applied to this transaction. A write result contains
            an ``update_time`` field.

        Raises:
            ValueError: If no transaction is in progress.
        """
        if not self.in_progress:
            raise ValueError(_CANT_COMMIT)

        commit_response = _commit_with_retry(self._client, self._write_pbs, self._id)

        self._clean_up()
        return list(commit_response.write_results)