Fix (gpxq): tensor unpacking and Cholesky stabilization

[i-colbert]

Reason for this PR

Stabilizing Cholesky decomposition in GPTQ and generalizing the quant tensor unpacking to work with other quant tensors.

Changes Made in this PR

• Centralized quant tensor unpacking in GPxQ base class and generalized to use cache_class attribute of input_quant
• Added another input to GPTQ.single_layer_update(...) to stabilize the Cholesky decomposition of the inverse Hessian

Testing Summary

N/A

Risk Highlight

• This PR includes code from another work (please detail).
• This PR contains API-breaking changes.
• This PR depends on work in another PR (please provide links/details).
• This PR introduces new dependencies (please detail).
• There are coverage gaps not covered by tests.
• Documentation updates required in subsequent PR.

Checklist

• Code comments added to any hard-to-understand areas, if applicable.
• Changes generate no new warnings.
• Updated any relevant tests, if applicable.
• No conflicts with destination dev branch.
• I reviewed my own code changes.
• Initial CI/CD passing.
• 1+ reviews given, and any review issues addressed and approved.
• Post-review full CI/CD passing.