[FSDP2] Issues in Wrap Policy and Mixed Precision (#3528)
* fix fsdp2 wrap policy
* nn.Module doesn't have the dtype attribute
* Revert "nn.Module doesn't have the dtype attribute"
This reverts commit 513c7892876f81ec76ce32bcdce83bfe8556491d.
* Fix dtype handling in fsdp2_prepare_model to accommodate nn.Module without dtype attribute
* fix format problem
