About zero_grad()

[whsun21]

I found the call of self.optimizer.zero_grad() and self.zero_grad() after self.meta_update(loss=losses['loss']), what is the purpose of them? It seems like self.optimizer.zero_grad() was already called in self.meta_update(loss=losses['loss']). As for self.zero_grad(), I couldn't get the aim of it. Could you please explain them? Thanks a lot!