Prevent using anything other than intrusive_ptr for Future (#58421)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58421
Here I make it impossible to create Futures that do not use intrusive_ptr, by making the constructor private. This makes it safer (by "forcing" people to do the right thing) and prevents a proliferation of new shared_ptrs or of accidental copies/moves.
ghstack-source-id: 129567047
Test Plan: CI
Reviewed By: mrshenli
Differential Revision: D28474484
fbshipit-source-id: 82c487e1bb7c27a2e78cb5d594e00e54c752bf09
