fix(utils): include original cause in _LazyModule ModuleNotFoundError

[Sayyam-Akram]

What is the purpose of this PR?

Fixes #47893

If an optional module like accelerate or sentencepiece is not installed in your system, and the import fails, for example when using MistralForSequenceClassification, then _LazyModule.__getattr__ will catch the underlying ModuleNotFoundError and raise a generic error message. This obscures what is really needed from the user.

This PR enhances the _LazyModule.__getattr__ function with the original exception information (Caused by: {e}). This provides instant, unmistakable diagnostic information of what package is missing.

I've also written a unit test in tests/utils/test_import_utils.py to make sure that exception causes are preserved.

Code Agent Policy

• (Only for the first time) I confirm that this PR description and code is not written by an LLM or code agent

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Have read the contributor guideline and the Pull Request checks?
• Has a Github issue or the forum been consulted/approved? Please add a link to it if that's the case. (Fixes ModuleNotFoundError: Could not import module 'MistralForSequenceClassification' #47893)
• Did you make sure to update the documentation with your changes according to the guidelines?
• Did you write any new necessary tests?

Who can review?

• model loading / import_utils: @Cyrilvallez @ArthurZucker

[github-actions]

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CI recap

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[Rocketknight1]

this is the most code agent code i've ever seen in my life and you're blocked