Cleanup learn model weights disturbed during LRFinder exploration.

def on_train_end(self, **kwargs:Any)->None:
        "Cleanup learn model weights disturbed during LRFinder exploration."
        self.learn.load('tmp', purge=False)
        if hasattr(self.learn.model, 'reset'): self.learn.model.reset()
        for cb in self.callbacks:
            if hasattr(cb, 'reset'): cb.reset()
        print('LR Finder is complete, type {learner_name}.recorder.plot() to see the graph.')