Update the params from master to model and zero grad.

def on_step_end(self, **kwargs:Any)->None:
        "Update the params from master to model and zero grad."
        #Zeros the gradients of the model since the optimizer is disconnected.
        self.learn.model.zero_grad()
        #Update the params from master to model.
        master2model(self.model_params, self.master_params, self.flat_master)