Initialize optimizer and learner hyperparameters.

def on_train_begin(self, pbar, **kwargs:Any)->None:
        "Initialize optimizer and learner hyperparameters."
        setattr(pbar, 'clean_on_interrupt', True)
        self.learn.save('tmp')
        self.opt = self.learn.opt
        self.opt.lr = self.sched.start
        self.stop,self.best_loss = False,0.
        return {'skip_validate': True}