def after_training_exp(self, strategy, **kwargs):
        """
        Save a copy of the model after each experience and
        update self.prev_classes to include the newly learned classes.
        """
        if _at_task_boundary(strategy.experience, before=False):
            strategy.model.features = None
            self.prev_model = copy.deepcopy(strategy.model)

        Save a copy of the model after each experience and
        update self.prev_classes to include the newly learned classes.
        
avalanche/training/plugins/feature_distillation.py