def callback(self, i, named_losses, overall_loss, grads, wrt_value):
raise NotImplementedError()<EOL>
This function will be called within [optimizer.minimize](vis.optimizer.md#minimize).

        Args:
            i: The optimizer iteration.
            named_losses: List of `(loss_name, loss_value)` tuples.
            overall_loss: Overall weighted loss.
            grads: The gradient of input image with respect to `wrt_value`.
            wrt_value: The current `wrt_value`.