[FSDP][Example] Fix FSDP example for use_orig_params=False

[SumanthRH]

What does this PR do?

Fixes the FSDP example for use_orig_params=False. I recently started using FSDP via Accelerate and hit some roadblocks due to rough edges in documentation and examples. Some improvements in this PR:

• use_orig_params=False should be the recommended route all the time (full param/ lora), since it's typically more efficient.
• use_orig_params=False requires model preparation before optimizer instantiation. This is a silent failure - I noticed that loss didn't decrease in this case. I believe it is best if .prepare has extra validation for this and raise an Error to the user. (Might do this in a separate PR if I get time). The example prepares everything together because it assumes use_orig_params is True from this PR: fsdp refactoring #2177
• Documentation has removed this caveat with use_orig_params. I found a note from an older version so I brought it back.
• SHARD_GRAD_OP is a bit different from Zero Stage 2, this PR adds a tiny note.

Fixes # (issue)

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cc @muellerzr

[BenjaminBossan]

• use_orig_params=False requires model preparation before optimizer instantiation. This is a silent failure - I noticed that loss didn't decrease in this case.

Nice catch, I didn't know about that.

[HuggingFaceDocBuilderDev]

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