[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)

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.

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]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the contributing guidelines are likely to be ignored.

[SumanthRH]

not stale

[muellerzr]

@SumanthRH what this hints at is when we do accelerator.prepare() we need to:

1. Find the model (there are many cases of us doing so)
2. Prepare said model in FSDP before everything else

Would you be willing to tackle it this way instead? It's critical we maintain the .prepare() only once workflow. Else I can dig into it

[SumanthRH]

Yes that's much better behaviour. Let me take a stab at it this week.