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)

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• [ ] Did you write any new necessary tests?

Who can review?

@muellerzr 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.

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