fix: protect pre-existing layers attribute from hqq overwrite

[gsprochette]

Description

This PR adds protection arround the layers attribute in hqq_diffusers.

hqq_diffusers declares a list of modules in a layers attribute to replicate the behavior of the transformers architecture. However, this creates conflicts with diffusers using the layers name, e.g. z-image.

This PR adds a context that temporarily renames a pre-existing layers attribute and restores it after quantization.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Unit tests, notebook available.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Additional Notes

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