support fine-tuned tinybert and mobilebert

[jue-zhang]

What does this PR do?

To support other small bert-like models (e.g., tinyBERT, mobileBERT) for LLMLingua2

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Was this discussed/approved via a Github issue? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

@iofu728, @QianhuiWu

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