FSDP2: Activation-checkpoint modules directly matching auto-wrap policy, not those whose parent module matches policy

[excepshenal]

What does this PR do?

Addresses #3897 :

1. FSDP2: Activation-checkpoint modules directly matching auto-wrap policy, not those whose parent module matches policy (introduced by [FSDP2] Refactor + FP8 #3585 , not sure about the context of why)
2. Nit: replace duplicate fsdp2_prepare_auto_wrap_policy with FullyShardedDataParallelPlugin.set_auto_wrap_policy, and delete extra unnecessary calls to set_auto_wrap_policy

Confirmed in test runs of llama-70b training that this fixes AC to checkpoint modules matching auto-wrap policy rather than those whose parent module matches the policy. Compare before:

to after:

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.