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:

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[excepshenal]

Just a light bump @SunMarc or whoever is best to tag for this! Thank you