Fix FSDP auto_wrap using characters instead of full str for layers

[muellerzr]

What does this PR do?

Recent release broke FSDP for models like llama that define a set of layers to split by. It'd parse the string as a tuple of characters instead. This PR fixes it + adjusts the tests to be more strict (tests bert and llama for auto wrap policy)

Fixes critical issue of FSDP being broken on the release.

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@SunMarc

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[SunMarc]

LGTM ! Thanks for fixing !

[anhuong]

Thank you so much for this fix! When can we expect to have this change in a release?

[muellerzr]

@anhuong we're waiting on one other PR to pass its tests, then I'll be doing a patchfix today