Make LazyGraphExecutor extensible (#87218)
Add `LazyGraphExecutor` to backend interface so that its is extensible by a vendor backend.
I've made some preliminary methods virtual. Not sure if we want to make all methods in `LazyGraphExecutor` virtual.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87218
Approved by: https://github.com/wconstab, https://github.com/alanwaketan
