refactor: simplify parameter freezing in modeling_base.py

[Ki-Seki]

What does this PR do?

Refactor the parameter freezing logic to simplify the handling of shared and unshared parameters. The original implementation was confusing; this version is clearer. Additionally, the current shared approach merely performs a deep copy—consider adding a TODO to switch to a shallow copy for shared layers in the future to save GPU memory.

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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 write any new necessary tests?

Who can review?

@lvwerra @lewtun @albertvillanova @qgallouedec

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[Ki-Seki]

@qgallouede, thank you for running checks. I found that two CI checks failed, which may be some issues with the main branch. This PR is just a simple refactor.🤗