Begin the transaction.

        Args:
            retry_id (Optional[bytes]): Transaction ID of a transaction to be
                retried.

        Raises:
            ValueError: If the current transaction has already begun.

def _begin(self, retry_id=None):
        """Begin the transaction.

        Args:
            retry_id (Optional[bytes]): Transaction ID of a transaction to be
                retried.

        Raises:
            ValueError: If the current transaction has already begun.
        """
        if self.in_progress:
            msg = _CANT_BEGIN.format(self._id)
            raise ValueError(msg)

        transaction_response = self._client._firestore_api.begin_transaction(
            self._client._database_string,
            options_=self._options_protobuf(retry_id),
            metadata=self._client._rpc_metadata,
        )
        self._id = transaction_response.transaction