fix(chapter-2): add missing loss_function to the test function

[imteekay]

- def test(dataloader, model):
+ def test(dataloader, model, loss_function):
    size = len(dataloader.dataset)
    num_batches = len(dataloader)
    model.eval()  # Set the model to evaluation mode
    test_loss, correct = 0, 0
    with torch.no_grad():
        for X, y in dataloader:
            pred = model(X)
            test_loss += loss_function(pred, y).item()
            correct += (pred.argmax(1) == y).type(torch.float).sum().item()
    test_loss /= num_batches
    correct /= size
    print(f"Test Error: \n Accuracy: {(100*correct):>0.1f}%, Avg loss: {test_loss:>8f} \n")

# Evaluate the model
- test(test_loader, model)
+ test(test_loader, model, loss_function)