fix fsdp checkpointing issues
# What does this PR do?
1. FSDP loading now return the `load_result` to be given to `_issue_warnings_after_load`. Should be merged after https://github.com/huggingface/accelerate/pull/1745
2. Earlier it was wrongly saving `optimizer.pt` with FSDP, fixed it.
_The documentation is not available anymore as the PR was closed or merged._