Util for writing HuggingFace save_pretrained from a composer checkpoint

[dakinggg]

What does this PR do?

Adds a basic util function to load a composer checkpoint of a HuggingFaceModel and write out the config.json and pytorch_model.bin that are expected in a huggingface pretrained folder.

What issue(s) does this change relate to?

CO-1445

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[nik-mosaic]

Seems reasonable and helpful. Approved.

[mvpatel2000]

LGTM!