Support loading custom models (trust_remote_code=True) in offline mode from local

[VladKha]

What does this PR do?

Fixes #34855
And should fix UKPLab/sentence-transformers#2613 (will close separately)

• Now custom models (models that require trust_remote_code=True) can be fully saved with save_pretrained() and able to be loaded with from_pretrained() in offline mode
• All required code files are now saved in the local dir during save_pretrained(). And during loading with from_pretrained() in offline mode (even on a new fresh machine) it won't try to reach out to HF hub since all files will be self-contained in local dir
• Adds a couple of tests for corresponding behavior

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline, Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
  Offline mode doesn't work with models that require trust_remote_code=True #34855 (comment)
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and here are tips on formatting docstrings.
  No need to update the docs IMHO, since it adds expected behavior according to current docs
• Did you write any new necessary tests?

Who can review?

@Rocketknight1
@ArthurZucker

[github-actions]

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[VladKha]

@Rocketknight1, run_tests has been running for 5 days now. Are you aware if this is expected behavior?