Take a step in lr,mom sched, start next stepper when the current one is complete.

def on_batch_end(self, train, **kwargs:Any)->None:
        "Take a step in lr,mom sched, start next stepper when the current one is complete."
        if train:
            if self.idx_s >= len(self.scheds): return {'stop_training': True, 'stop_epoch': True}
            sched = self.scheds[self.idx_s]
            for k,v in sched.items(): self.opt.set_stat(k, v.step())
            if list(sched.values())[0].is_done: self.idx_s += 1