Default FSDP weights merge to safetensors

[helloworld1]

What does this PR do?

The new accelerate merge-weights should default use safetensors format. However, it is using bin format.

$ accelerate merge-weights checkpoint-2000/pytorch_model_fsdp_0/ checkpoint-2000-merged-weight

$ ls checkpoint-2000-merged-weight
pytorch_model.bin

After this change it is now model.safetensors

Fixes # (issue)

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@muellerzr
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[HuggingFaceDocBuilderDev]

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[SunMarc]

LGTM !

[muellerzr]

Thanks for the fix!