//  [python]
def train(args, model, train_loader, optimizer, epoch, device):
    start_time = datetime.now()

    criterion = nn.CrossEntropyLoss()

    losses = []
    top1_acc = []

    for i, (images, target) in enumerate(tqdm(train_loader)):
        images = images.to(device)
        target = target.to(device)

        # Step 1: compute per-sample-grads

        # To use vmap+grad to compute per-sample-grads, the forward pass
        # must be re-formulated on a single example.
        # We use the `grad` operator to compute forward+backward on a single example,
        # and finally `vmap` to do forward+backward on multiple examples.
        def compute_loss_and_output(weights, image, target):
            images = image.unsqueeze(0)
            targets = target.unsqueeze(0)
            output = functional_call(model, weights, images)
            loss = criterion(output, targets)
            return loss, output.squeeze(0)

        # `grad(f)` is a functional API that returns a function `f'` that
        # computes gradients by running both the forward and backward pass.
        # We want to extract some intermediate
        # values from the computation (i.e. the loss and output).
        #
        # To extract the loss, we use the `grad_and_value` API, that returns the
        # gradient of the weights w.r.t. the loss and the loss.
        #
        # To extract the output, we use the `has_aux=True` flag.
        # `has_aux=True` assumes that `f` returns a tuple of two values,
        # where the first is to be differentiated and the second "auxiliary value"
        # is not to be differentiated. `f'` returns the gradient w.r.t. the loss,
        # the loss, and the auxiliary value.
        grads_loss_output = grad_and_value(compute_loss_and_output, has_aux=True)
        weights = dict(model.named_parameters())

        # detaching weights since we don't need to track gradients outside of transforms
        # and this is more performant
        detached_weights = {k: v.detach() for k, v in weights.items()}
        sample_grads, (sample_loss, output) = vmap(grads_loss_output, (None, 0, 0))(
            detached_weights, images, target
        )
        loss = sample_loss.mean()

        for name, grad_sample in sample_grads.items():
            weights[name].grad_sample = grad_sample.detach()

        # Step 2: Clip the per-sample-grads, sum them to form grads, and add noise
        clip_and_accumulate_and_add_noise(
            model, args.max_per_sample_grad_norm, args.sigma
        )

        preds = np.argmax(output.detach().cpu().numpy(), axis=1)
        labels = target.detach().cpu().numpy()
        losses.append(loss.item())

        # measure accuracy and record loss
        acc1 = accuracy(preds, labels)

        top1_acc.append(acc1)

        # make sure we take a step after processing the last mini-batch in the
        # epoch to ensure we start the next epoch with a clean state
        optimizer.step()
        optimizer.zero_grad()

        if i % args.print_freq == 0:
            print(
                f"\tTrain Epoch: {epoch} \t"
                f"Loss: {np.mean(losses):.6f} "
                f"Acc@1: {np.mean(top1_acc):.6f} "
            )
    train_duration = datetime.now() - start_time
    return train_duration