Add iterator like functionality for DispatchKeySet (#44066)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44066
Add STL Input iterator to DispatchKeySet:
* Iterator is able to iterate from first not undefined DispatchKey
to NumDispatchKeys.
* Iterator is invalidated once underlying DispatchKeySet is invalidated
Note see http://www.cplusplus.com/reference/iterator/ for comparisons of
different iterators.
Test Plan: Imported from OSS
Reviewed By: ezyang
Differential Revision: D23611405
Pulled By: linux-jedi
fbshipit-source-id: 131b287d60226a1d67a6ee0f88571f8c4d29f9c3
