Register the `Hooks` on `self.modules`.

def on_train_begin(self, **kwargs):
        "Register the `Hooks` on `self.modules`."
        if not self.modules:
            self.modules = [m for m in flatten_model(self.learn.model)
                            if hasattr(m, 'weight')]
        self.hooks = Hooks(self.modules, self.hook)