Allow `Trainer.get_optimizer_cls_and_kwargs` to be overridden (#31875)
* Change `Trainer.get_optimizer_cls_and_kwargs` to `self.`
* Make `get_optimizer_cls_and_kwargs` an instance method
* Fixing typo
* Revert `get_optimizer_cls_and_kwargs` to staticmethod
* restore newline to trainer.py eof