Add Slot type to abstract the raw pointers being used for slots. (#18226)

Add Slot type to abstract the raw pointers being used for slots. (#18226)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18226
ghimport-source-id: b9ec8651212875b30971cc6859d2ddec6559ae3a

If modules become first-class IValues, then the slots will no longer be raw pointers but (IValue, index) pairs. This commit inserts the Slot abstraction so that this change can be made in later patches.

Stack from [ghstack](https://github.com/ezyang/ghstack):
* **#18226 Add Slot type to abstract the raw pointers being used for slots.**

Differential Revision: D14542022

fbshipit-source-id: b81d7f4334c983d663e7551bda82df43680d7c5f