Command line argument of learning rate scheduler

[hlh981029]

#120

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[hlh981029]

https://github.com/alibaba/EasyNLP/blob/169bf0a706985a06fc2598960fdafc8c3be64944/easynlp/core/trainer.py#L325-L334

这里的逻辑我不太理解，比如，self._optimizer 是 BertAdam 的时候 self._lr_scheduler 为 None，这里是需要调用 self._optimizer.step() 的。