A clean PyTorch training loop with validation and checkpoints

                  import torch

best_val_loss = float('inf')

for epoch in range(1, num_epochs + 1):
    model.train()
    train_loss = 0.0
    for batch in train_loader:
        inputs, targets = [item.to(device) for item in batch]
        optimizer.zero_grad(set_to_none=True)
        outputs = model(inputs)
        loss = criterion(outputs, targets)
        loss.backward()
        torch.nn.utils.clip_grad_norm_(model.parameters(), max_norm=1.0)
        optimizer.step()
        train_loss += loss.item() * inputs.size(0)

    model.eval()
    val_loss = 0.0
    with torch.no_grad():
        for batch in valid_loader:
            inputs, targets = [item.to(device) for item in batch]
            outputs = model(inputs)
            loss = criterion(outputs, targets)
            val_loss += loss.item() * inputs.size(0)

    val_loss /= len(valid_loader.dataset)
    if val_loss < best_val_loss:
        best_val_loss = val_loss
        torch.save({'model_state': model.state_dict()}, 'best_model.pt')

    scheduler.step(val_loss)
    print(f'epoch={epoch} val_loss={val_loss:.4f}')