distillation loss is zero in LwF plugin for multi task module

[matkowski-voy]

Hi,

It is related to #https://github.com/ContinualAI/avalanche/issues/1116 I think there is still something wrong because the dist_loss is zero. It seems that for the case where isinstance(self.prev_model, MultiTaskModule) is True, task_id in self.prev_classes: is always False and thus dist_loss is always zero.

I think because the task_id is int whereas in self.prev_classes, the keys are str

Please let me know if that's the case or I am missing something there.

Thanks, Woj