Add small util to enable FSDP offloading quickly

[muellerzr]

What does this PR do?

Enables users to quickly do:

from accelerate.utils import enable_fsdp_ram_efficient_loading
enable_fsdp_ram_efficient_loading()

model = AutoModel.from_*

(Also some tests got commented out when I was toying with things, hence the odd diff)

Fixes # (issue)

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@BenjaminBossan

Other info

Note that we have self.ram_efficient_loading in the FSDP Plugin. For clarity it should be cpu_ram_efficient_loading and also this was only added in the prior PR, so it's fine.

[HuggingFaceDocBuilderDev]

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[BenjaminBossan]

Thanks for updating the functionality, LGTM. Just a small nit, feel free to ignore.

[BenjaminBossan]

Nit: "in the environment" might not really be helpful for a user.

[muellerzr]

After having Stas complain so much about magical env variables being set, I'd rather over document it hah