Following is the code I am working on:
def train_epoch(train_loader, model, loss_fn, optimizer, cuda, log_interval, metrics, step=None):
    for metric in metrics:
        metric.reset()

    model.train()
    losses = []
    total_loss = 0


    for batch_idx, ((x0, x1), y) in enumerate(train_loader):
        y_true = y
        x0, x1, y_true = x0.cuda(), x1.cuda(), y.cuda()
 
        optimizer.zero_grad()
        output1, output2 = model(x0, x1)

        #p_dist = torch.nn.PairwiseDistance(keepdim=True)
        p_dist = torch.nn.CosineSimilarity(dim=1, eps=1e-08)

        dy = p_dist(output1, output2)
        dy = torch.nan_to_num(dy)
        y_true = torch.nan_to_num(y_true)


        '''2 lines indicated the normalization of dy to 0 and 1 by dividing it with max value'''

        maximum_dy = torch.max(dy)
        maximum_dy = torch.nan_to_num(maximum_dy)
        dy = dy / maximum_dy

        maximum_y_true = torch.max(y_true)
        maximum_y_true = torch.nan_to_num(maximum_y_true)

        y_true = y_true / maximum_y_true

        #dy = torch.squeeze(dy, 1)
        input_dy = torch.empty(dy.size(0), 2)

        input_dy[:, 0] = 1 - dy
        input_dy[:, 1] = dy
   
        y_true_2 = torch.zeros(dy.size(0), 2)
        y_true_2[range(y_true_2.shape[0]), y_true.long()] = 1
        m = nn.Sigmoid()
        loss = loss_fn(m(input_dy), y_true_2)

        loss.backward()
        optimizer.step()

        losses.append(loss.item())
        total_loss += loss.item()

        input_dy_metric = torch.round(input_dy)

        for metric in metrics:
            metric(input_dy_metric, y_true_2)
            metric.total += y_true_2.shape[0]

        if batch_idx % log_interval == 0:
            message = 'Train: [{}/{} ({:.0f}%)]\tLoss: {:.6f}'.format(
                batch_idx, len(train_loader),
                100. * batch_idx / len(train_loader), np.mean(losses))
            for metric in metrics:
                message += '\t{}: {}'.format(metric.name(), metric.value())

            losses = []


    total_loss /= (batch_idx + 1)
    return total_loss, metrics


def test_epoch(val_loader, model, loss_fn, cuda, metrics, log_interval):
    with torch.no_grad():
        for metric in metrics:
            metric.reset()
        model.eval()
        val_loss = 0
        losses = []
        for batch_idx, ((x0, x1), label) in enumerate(val_loader):
            x0, x1, y_true = x0.cuda(), x1.cuda(), label.cuda()

            output1, output2 = model(x0, x1)

            #p_dist = torch.nn.PairwiseDistance(keepdim=True)
            p_dist = torch.nn.CosineSimilarity(dim=1, eps=1e-08)

            dy = p_dist(output1, output2)
            dy = torch.nan_to_num(dy)
            y_true = torch.nan_to_num(y_true)
      
            '''2 lines indicated the normalization of dy to 0 and 1 by dividing it with max value'''

            maximum_dy = torch.max(dy)
            maximum_dy = torch.nan_to_num(maximum_dy)
            dy = dy / maximum_dy

            maximum_y_true = torch.max(y_true)
            maximum_y_true = torch.nan_to_num(maximum_y_true)

            y_true = y_true / maximum_y_true

            #dy = torch.squeeze(dy, 1)

            'Output tensor of dimension [4,2] and input tensor of dimension [4,2] to BCE loss function'
            input_dy = torch.empty(dy.size(0), 2)
            input_dy[:, 0] = 1 - dy
            input_dy[:, 1] = dy
          

            y_true_2 = torch.zeros(dy.size(0), 2)
        
            y_true_2[range(y_true_2.shape[0]), y_true.long()] = 1
         
            m = nn.Sigmoid()
            loss = loss_fn(m(input_dy), y_true_2)
            losses.append(loss.item())
            val_loss += loss.item()

            input_dy_metric = torch.round(input_dy)
            for metric in metrics:
                metric(input_dy_metric, y_true_2)
                metric.total += y_true_2.shape[0]

            if batch_idx % log_interval == 0:
                message = 'Test: [{}/{} ({:.0f}%)]\tLoss: {:.6f}'.format(
                    batch_idx, len(val_loader),
                    100. * batch_idx / len(val_loader), np.mean(losses))
                for metric in metrics:
                    message += '\t{}: {}'.format(metric.name(), metric.value())

                #print(message)
                losses = []

    val_loss /= (batch_idx + 1)
        
    return val_loss, metrics

loss_fn = torch.nn.BCELoss()
#loss_fn = torch.nn.ContrastiveLoss(pos_margin=0, neg_margin=1)
lr = 1e-4
optimizer = optim.Adam(model.parameters(), lr=lr)

scheduler = lr_scheduler.StepLR(optimizer, 8, gamma=0.1, last_epoch=-1)
n_epochs = 80
log_interval = 1

fit(train_loader, test_loader, model, loss_fn, optimizer, scheduler, n_epochs, cuda, log_interval, [metrics.Binary_accuracy()])

Fit function:
def fit(train_loader, val_loader, model, loss_fn, optimizer, scheduler, n_epochs, cuda, log_interval, metrics,
        start_epoch=0, message=None) -> object:
  
    for epoch in range(0, start_epoch):
        scheduler.step()

    for epoch in range(start_epoch, n_epochs):
        scheduler.step()

        # Train stage
        train_loss, metrics = train_epoch(train_loader, model, loss_fn, optimizer, cuda, log_interval, metrics)
       
        message = 'Epoch: {}/{}. Train set: Average loss: {:.4f}'.format(epoch + 1, n_epochs, train_loss)
        for metric in metrics:
            message += '\t{}: {}'.format(metric.name(), metric.value())

        val_loss, metrics = test_epoch(val_loader, model, loss_fn, cuda, metrics, log_interval)
        val_loss /= len(val_loader)

        message += '\nEpoch: {}/{}. Test set: Average loss: {:.4f}'.format(epoch + 1, n_epochs,
                                                                                 val_loss)
        for metric in metrics:
            message += '\t{}: {}'.format(metric.name(), metric.value())

        print(message)