After checking GRU implement, I think I can probably find answer from the implement of self._backend.RNN
        func = self._backend.RNN(
            self.mode,
            self.input_size,
            self.hidden_size,
            num_layers=self.num_layers,
            batch_first=self.batch_first,
            dropout=self.dropout,
            train=self.training,
            bidirectional=self.bidirectional,
            batch_sizes=batch_sizes,
            dropout_state=self.dropout_state
        )

But how can see these code??