Update UTILS.md

[JamePeng]

Since part of the previous readme.md content has been moved to utils.md, resubmit

Update FullModelHFCheckpointer import to align with TorchTune API changes.

https://github.com/pytorch/torchtune/blob/7d29c21149ace4be9fdc2709a03c0a21ba191010/torchtune/training/__init__.py#L41

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