slots with explicit value/setValue make more sense in future patches (#18468)

slots with explicit value/setValue make more sense in future patches (#18468)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18468
ghimport-source-id: d4b41c521f2269a695e03c8e7d05d5542731ee48

Stack from [ghstack](https://github.com/ezyang/ghstack):
* #18469 Create Object that represents a Module
* **#18468 slots with explicit value/setValue make more sense in future patches**
* #18467 Make Object hold its ClassType
* #18379 Enforce single parent for script submodules
* #18378 Unify namespace of script::Module
* #18314 Add ability to specialize class types to ArgumentSpec
* #18226 Add Slot type to abstract the raw pointers being used for slots.

Reviewed By: suo

Differential Revision: D14613509

fbshipit-source-id: 9f2208d0efd01465c78cebdc3e8365a9e0adf9ff