Update docs to mention CUDA support for Future (#50048)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50048
To reflect the many changes introduced recently.
In my mind, CUDAFuture should be considered a "private" subclass, which in practice should always be returned as a downcast pointer to an ivalue::Future. Hence, we should document the CUDA behavior in the superclass, even if it's CUDA-agnostic, since that's the interface the users will see also for CUDA-enabled futures.
ghstack-source-id: 128640983
Test Plan: Built locally and looked at them.
Reviewed By: mrshenli
Differential Revision: D25757474
fbshipit-source-id: c6f66ba88fa6c4fc33601f31136422d6cf147203
