[KV Cache][TurboQuant] Add official 3-bit and 4-bit grouped TurboQuant modes

[erhan1209]

Purpose

Implement the official-style TurboQuant KV-cache modes for turboquant_4bit and turboquant_3bit in vLLM, while preserving the existing legacy *_nc TurboQuant paths.

This PR does the following:

• adds canonical support for turboquant_4bit and turboquant_3bit cache dtypes
• ports grouped TurboQuant reference/layout/metadata utilities based on the sibling vllm-turboquant work
• wires grouped metadata-aware MSE+QJL-style key packing into the attention backend
• adds a grouped Triton key store path for the official modes on CUDA
• adds a grouped Triton key decode path for the official modes on CUDA
• keeps the existing uniform value format, with Triton-backed value packing on the grouped CUDA store path
• preserves legacy turboquant_4bit_nc, turboquant_3bit_nc, and turboquant_k3v4_nc behavior

This is intended to move the official TurboQuant modes materially closer to the reference/sibling implementation without regressing the existing legacy TurboQuant presets.

Duplicate-work check:

• I checked for obvious duplicate local work before preparing this branch.
• I am not aware of an existing open PR in this workspace that implements the same grouped official TurboQuant integration.
• If there is an upstream/open PR covering the same area, this branch differs by focusing on grouped official-mode integration while retaining the legacy *_nc path separately.

AI assistance:

• This PR was prepared with AI assistance.
• The submitting human is responsible for reviewing and validating all changed lines before merge.

Test Plan

I did not run tests for this PR.

Reason:

• the required uv / .venv environment described in AGENTS.md was not available in this workspace during implementation
• per project instructions, I did not fall back to bare system Python

Intended follow-up test plan once the environment is available:

• .venv/bin/python -m pytest tests/quantization/test_turboquant.py -v
• .venv/bin/python -m pytest tests/quantization/test_turboquant_reference.py -v

Test Result

Not run.

Current validation status:

• implementation completed by code inspection and integration work only
• no runtime validation, numerical validation, or performance benchmarking was performed in this workspace
• reviewers should treat this as an implementation-focused PR until the above test plan is executed

Notes

Documentation:

• No documentation files were updated in this PR.
• If these official TurboQuant modes are accepted as user-facing and supported, follow-up docs may be needed.

Release notes:

• Not updated in this PR.
• If maintainers consider this user-facing enough for release notes, that should be added before release.

[github-actions]

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[gemini-code-assist]

Code Review

This pull request implements official 3-bit and 4-bit TurboQuant KV cache presets, introducing a grouped layout that utilizes WHT-rotated Lloyd-Max quantization alongside 1-bit residual sign correction (QJL). The changes include new Triton kernels for grouped decoding and storage, a metadata system for high-precision indices, and updated configuration presets. Feedback identifies missing math imports in several Triton files and highlights performance optimization opportunities, specifically regarding redundant computations in the KV update path, inefficient bit-packing logic, and low hardware utilization due to small block sizes. A potential performance regression in the prefill path for QJL-enabled modes was also noted.

[erhan1209]

Solved issues.

[gaby]

@erhan1209 Is this a continuation to #38479 ?

[erhan1209]

@erhan1209 Is this a continuation to #38479 ?

I still preserved that legacy ncs. What I had in mind is more like full turboquant paper aligned implementation, feel free to point out your observations or suggestions.

[gaby]

@erhan1209 Is this a continuation to #38479 ?

I still preserved that legacy ncs. What I had in mind is more like full turboquant paper aligned implementation, feel free to point out your observations or suggestions.

Yeah, I see. Makes sense to me.