[LLMLingua2] Save model in huggingface style

[noiji]

What does this PR do?

Saves model in huggingface style, so that a user can directly load the trained model.

Fixes # (issue)

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• 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 make sure to update the documentation with your changes?
• Did you write any new necessary tests?

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[noiji]

@microsoft-github-policy-service agree

[QianhuiWu]

Revised model saving. Approved.