[FSDP] Relax post-backward assert (#89791)
This assert was accidentally made stricter when transitioning from per-FSDP-instance training state to per-handle training state. This PR relaxes it again, which should restore compatibility for some reentrant AC plus FSDP cases.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89791
Approved by: https://github.com/zhaojuanmao
