Make ScriptModule.training an attribute instead of a parameter (#19587)
Summary:
Stack from [ghstack](https://github.com/ezyang/ghstack):
* **#19587 [jit] Make ScriptModule.training an attribute instead of a parameter**
Remove the hack we had previously where `training` was a buffer
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19587
Differential Revision: D15502768
Pulled By: driazati
fbshipit-source-id: 3022f2d57ec6849868f9225d9bc2bfb7828cb318
