feat: layerwise decompression/compression support

[kylesayrs]

Summary

• Adds leave_decompressed parameter to decompress_module() — when False, strips all quantization metadata (scheme, status, qparams, forward hooks), enabling decompress-then-reapply workflows
• Adds allowed_modules parameter to apply_quantization_config() and _apply_kv_cache_scheme() to restrict quantization initialization to a subset of modules (e.g., a single decoder layer)

These APIs enable layerwise decompression and compression in the sequential pipeline, where each layer can be decompressed, recalibrated, and re-compressed independently.

Companion PR: llm-compressor (TBD) integrates these APIs into the sequential pipeline via layerwise_decompression / layerwise_compression data arguments

Test plan

• Verify decompress_module(leave_decompressed=True) preserves quantization metadata (existing behavior)
• Verify decompress_module(leave_decompressed=False) fully strips quantization
• Verify apply_quantization_config(allowed_modules=...) only quantizes specified modules
• Verify round-trip: decompress(leave_decompressed=False) → apply_quantization_config(allowed_modules) → calibrate → compress

🤖 Generated with Claude Code

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[mergify]

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @kylesayrs.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork