Add ability to specialize class types to ArgumentSpec (#18314)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18314
ghimport-source-id: 8cecb768d476ab19c9460f39c8f94a764e4cb052
Stack from [ghstack](https://github.com/ezyang/ghstack):
* **#18314 Add ability to specialize class types to ArgumentSpec**
* #18226 Add Slot type to abstract the raw pointers being used for slots.
Differential Revision: D14574395
fbshipit-source-id: cc3af6e56e9ae52990f4a1ad56ecceaa2d493577
