[RFC] Add warning about object-based collectives for GPU tensors to docs. (#97702)
Using GPU tensors in these collectives have caused SEVs, user
confusion, and slowness in the past. These APIs were only designed to
communicate arbitrary python objects, and GPU tensors should either be copied
to CPU first or use the regular collecitves. Add a warning indicating so.
Differential Revision: [D44435849](https://our.internmc.facebook.com/intern/diff/D44435849/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97702
Approved by: https://github.com/kumpera
