Hi,
I got the error when I use .backward(), but I don’t know how to fix it, could anybody help me, thanks a lot.
here is my train code:
def train(config, model, train_iter, dev_iter, test_iter):
    start_time = time.time()
    model.train()
    optimizer = torch.optim.Adadelta(model.parameters(),
                                     lr=config.learning_rate)

    scheduler = torch.optim.lr_scheduler.ExponentialLR(optimizer, gamma=0.9)
    loss_fct = CrossEntropyLoss()
    total_batch = 1  
    dev_best_loss = float('inf')
    last_improve = 0  
    flag = False  
    writer = SummaryWriter(log_dir=config.log_path + '/' +
                           time.strftime('%m-%d_%H.%M', time.localtime()))
    for epoch in range(config.num_epochs):
        print('Epoch [{}/{}]'.format(epoch + 1, config.num_epochs))
        for _, trains in enumerate(train_iter):
            model.zero_grad()
            left_trains, right_trains, (mid_trains, labels) = trains
            outputs = model(left_trains.cuda(config.cuda_id),
                            right_trains.cuda(config.cuda_id),
                            mid_trains.cuda(config.cuda_id))
            loss = loss_fct(outputs, labels.cuda(config.cuda_id))
            loss.backward(retain_graph=True)
            optimizer.step()
            optimizer.zero_grad()
            if total_batch % 10 == 0:
                true = labels.data.cpu()
                predic = torch.max(outputs.data, 1)[1].cpu()
                train_acc = metrics.accuracy_score(true, predic)
                dev_acc, dev_loss = evaluate(config, model, dev_iter)
                if dev_loss < dev_best_loss:
                    dev_best_loss = dev_loss
                    torch.save(model.state_dict(), config.save_path)
                    improve = '*'
                    last_improve = total_batch
                else:
                    improve = ''
                time_dif = get_time_dif(start_time)
                msg = 'Iter: {0:>6},  Train Loss: {1:>5.2},  Train Acc: {2:>6.2%},  Val Loss: {3:>5.2},  Val Acc: {4:>6.2%},  Time: {5} {6}'
                print(
                    msg.format(total_batch, loss.item(), train_acc, dev_loss,
                               dev_acc, time_dif, improve))
                writer.add_scalar("loss/train", loss.item(), total_batch)
                writer.add_scalar("loss/dev", dev_loss, total_batch)
                writer.add_scalar("acc/train", train_acc, total_batch)
                writer.add_scalar("acc/dev", dev_acc, total_batch)
                model.train()
            total_batch += 1
            if total_batch - last_improve > config.require_improvement:
                print("No optimization for a long time, auto-stopping...")
                flag = True
                break
        if flag:
            break
        scheduler.step()
    writer.close()
    test(config, model, test_iter)


def test(config, model, test_iter):
    # test
    model.load_state_dict(torch.load(config.save_path))
    model.eval()
    start_time = time.time()
    test_acc, test_loss, test_report, test_confusion = evaluate(config,
                                                                model,
                                                                test_iter,
                                                                test=True)
    msg = 'Test Loss: {0:>5.2},  Test Acc: {1:>6.2%}'
    print(msg.format(test_loss, test_acc))
    print("Precision, Recall and F1-Score...")
    print(test_report)
    print("Confusion Matrix...")
    print(test_confusion)
    time_dif = get_time_dif(start_time)
    print("Time usage:", time_dif)


def evaluate(config, model, data_iter, test=False):
    model.eval()
    loss_total = 0
    predict_all = np.array([], dtype=int)
    labels_all = np.array([], dtype=int)
    with torch.no_grad():
        for evals in data_iter:
            left_evals, right_evals, mid_data = evals
            mid_evals, labels = mid_data
            labels = labels.cuda(config.cuda_id)
            outputs = model(left_evals.cuda(config.cuda_id),
                            right_evals.cuda(config.cuda_id),
                            mid_evals.cuda(config.cuda_id))
            loss = F.cross_entropy(outputs, labels)
            loss_total += loss
            labels = labels.data.cpu().numpy()
            predic = torch.max(outputs.data, 1)[1].cpu().numpy()
            labels_all = np.append(labels_all, labels)
            predict_all = np.append(predict_all, predic)

    acc = metrics.accuracy_score(labels_all, predict_all)
    if test:
        class_list = [int(i) for i in config.class_list.split()]
        report = metrics.classification_report(labels_all,
                                               predict_all,
                                               target_names=class_list,
                                               digits=4)
        confusion = metrics.confusion_matrix(labels_all, predict_all)
        return acc, loss_total / len(data_iter[2]), report, confusion
    return acc, loss_total / len(data_iter[2])

here is my model:
class LR_CNN(nn.Module):
    """
    Input shape:
        4D tensor with shape: (batch, K, pad_size, embed_dim)
     Output shape
        3D tensor with shape: (batch, K, len(filter_sizes)*num_filters)
    """
    def __init__(self, config) -> None:
        super(LR_CNN, self).__init__()
        filter_sizes = [int(i) for i in config.filter_sizes.split()]
        self.conv = nn.ModuleList([
            nn.Conv3d(1, config.num_filters, (1, i, config.embed))
            for i in filter_sizes
        ])
        self.relu = nn.ReLU()

    def conv_and_activate(self, x, conv):
        out = conv(x).squeeze(-1)
        out = F.max_pool2d(out, (1, out.size(3)))
        out = self.relu(out)
        out = out.squeeze(-1)
        out = out.permute(0, 2, 1)
        return out

    def forward(self, x):
        x = x.unsqueeze(1)
        out = torch.cat(
            [self.conv_and_activate(x, conv) for conv in self.conv], dim=2)
        return out


class Mid_CNN(nn.Module):
    """
    Input shape:
        3D tensor with shape: (batch, pad_size, embed_dim)
     Output shape
        2D tensor with shape: (batch, len(filter_sizes)*num_filters)
    """
    def __init__(self, config) -> None:
        super(Mid_CNN, self).__init__()
        filter_sizes = [int(i) for i in config.filter_sizes.split()]
        self.conv = nn.ModuleList([
            nn.Conv2d(1, config.num_filters, (i, config.embed))
            for i in filter_sizes
        ])
        self.relu = nn.ReLU()
        self.fc = nn.Linear(
            len(filter_sizes) * config.num_filters,
            len(filter_sizes) * config.num_filters)

    def conv_and_activate(self, x, conv):
        out = conv(x).squeeze(-1)
        out = F.max_pool1d(out, out.size(2))
        out = self.relu(out)
        return out.squeeze(-1)

    def forward(self, x):
        out = torch.cat(
            [self.conv_and_activate(x.unsqueeze(1), conv) for conv in self.conv], dim=1)
        out = self.fc(out)
        out = self.relu(out)
        return out.unsqueeze(1)


class LSTM(nn.Module):
    """
    Input shape:
        3D tensor with shape: (batch, K, len(filter_sizes)*num_filters)
     Output shape
        3D tensor with shape: (batch, K, num_directions * hidden_size)
    """
    def __init__(self, config) -> None:
        super(LSTM, self).__init__()
        input_size = len(config.filter_sizes.split()) * config.num_filters
        self.lstm = nn.LSTM(input_size,
                            config.hidden_size,
                            dropout=config.drop_out,
                            bidirectional=True,
                            batch_first=True,
                            num_layers=config.num_layers)
        # self.fc = nn.Linear(input_size, 2*config.hidden_size)

    def forward(self, x):
        if len(x.size()) == 2:
            x = x.unsqueeze(1)
        out, _ = self.lstm(x)
        # out = self.fc(x)
        return out


class Attention(nn.Module):
    """
    Input shape:
        3D tensor with shape: (batch, K, features)
     Output shape
        2D tensor with shape: (batch, features)
    """
    def __init__(self, config) -> None:
        super(Attention, self).__init__()
        self.w = nn.Parameter(torch.Tensor(config.hidden_size * 2).cuda(config.cuda_id))
        self.b = nn.Parameter(torch.Tensor(config.K).cuda(config.cuda_id))
        self.u = nn.Parameter(torch.Tensor(config.K, config.K).cuda(config.cuda_id))
        self._creat_weight()

    def _creat_weight(self, mean=0.0, std=0.05):
        self.w.data.normal_(mean, std)
        self.u.data.normal_(mean, std)

    def forward(self, x):
        uit = torch.matmul(x, self.w)
        uit += self.b
        uit = torch.matmul(uit, self.u)
        uit = torch.tanh(uit)
        uit = torch.exp(uit)
        ait = torch.sum(uit, dim=1).unsqueeze(1)
        uit = torch.div(uit, ait).unsqueeze(2)
        res = x * uit
        return torch.sum(res, dim=1)


class CBA(nn.Module):

    def __init__(self, config) -> None:
        super(CBA, self).__init__()
        self.lr_cnn = LR_CNN(config)
        self.mid_cnn = Mid_CNN(config)
        self.lr_lstm = LSTM(config)
        self.mid_lstm = LSTM(config)
        self.lr_attention = Attention(config)
        self.fc = nn.Linear(2 * config.hidden_size, 2 * config.hidden_size)
        self.fc1 = nn.Linear(6 * config.hidden_size, 2)
        self.relu = nn.ReLU()

    def forward(self, left, right, mid):
        l_out = self.lr_cnn(left)
        r_out = self.lr_cnn(right)
        mid_out = self.mid_cnn(mid)
        l_out = self.lr_lstm(l_out)
        r_out = self.lr_lstm(r_out)
        mid_out = self.mid_lstm(mid_out)
        mid_out = self.fc(mid_out)
        mid_out = self.relu(mid_out).squeeze(1)
        l_out = self.lr_attention(l_out)
        r_out = self.lr_attention(r_out)
        out = torch.cat((torch.cat((l_out, mid_out), dim=1), r_out), dim=1)
        out = self.fc1(out)
        out = F.softmax(out, dim=-1)
        return out

thanks