[1.13] Mention optim_input future BC breakage (#85963)
We should remove this arg when release after 1.13 rolls around, enhance warning to indicate it will be gone. We can do this as FSDP is still beta and can be BC breaking until we stabilize the API.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85963
Approved by: https://github.com/awgu
