fixes to properly shard FSDP across cpu and meta for cpu_efficient_loading for prequantized 4bit
Author: winglian
Pull request: yes
Labels: name: PyTorch FSDP

# What does this PR do?

When using FSDP cpu_efficient_loading, transformers is still loading all the parameters into cpu. This fixes it so rank 0 is properly offloading to cpu and other ranks to meta. I was able to load the `hugging-quants/Meta-Llama-3.1-405B-BNB-NF4-BF16` model with ~240GiB system RAM instead of the previous 1.6TB RAM.

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Fixes # (issue)


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Comments:
Thanks for helping out with this @winglian, much appreciated!

cc @muellerzr @SunMarc 

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