Migrate from shared_ptr to intrusive_ptr for Future (#57636)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57636
The "preferred" pointer holder for Future is `intrusive_ptr` (e.g., `then` returns an `intrusive_ptr`, `toFuture` returns `intrusive_ptr`, ...). However in RPC we often wrap it with `shared_ptr`. This probably dates back to when we had a separate Future type, before the merge.
At the boundary between RPC and JIT this difference becomes a bit annoying, as conversions between the pointer types are needed. I think it would be simpler and more consistent to always use `intrusive_ptr`, also in RPC.
This PR was produced mainly by find-and-replace, plus a couple of manual fixes.
ghstack-source-id: 128296581
Test Plan: CI
Reviewed By: pritamdamania87
Differential Revision: D28187972
fbshipit-source-id: d4609273a1550b4921910e85d2198e02f31c905b
