Merge CUDAFuture into ivalue::Future (#57052)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57052
This PR caps a stack whose goal was to merge CUDAFuture into ivalue::Future. CUDAFuture used to be a subclass of ivalue::Future, which was already pretty good, but it meant that in several places we needed `#ifdef`s or registries in order to create the right type of class, which was annoying. We've made CUDAFuture device-agnostic, by using generic helpers, so that it doesn't depend on CUDA. Now all its code can be inserted into ivalue::Future.
This PR does this very naively, by copy-pasting CUDAFuture's code into the (previously empty) virtual methods of ivalue::Future. This helps ensure the correctness of this PR, as it's straightforward to see it behaves exactly like before. However we probably want to polish it a bit later to iron out so wrinkles.
ghstack-source-id: 127713138
(Note: this ignores all push blocking failures!)
Test Plan: CI
Reviewed By: mrshenli
Differential Revision: D28036829
fbshipit-source-id: 3e5b16402f5dc245c1fcb9d7bf06db64dcb0d2a3
