[Bugfix] Fix hybrid KV manager for quantized per-token-head KV cache

[lesj0610]

summary

this pr fixes hybrid kv manager correctness for quantized per-token-head kv cache.

main changes are:

• preserve kv_quant_mode when hybrid kv specs are unified with hybrid manager disabled
• allow page-size unification through a common multiple when hybrid page sizes are not directly divisible
• use a common block-size divisor (gcd) for scheduler-side block size after hybrid kv groups are formed

this is a bugfix for already merged per-token-head kv support, and also prepares the same hybrid path for the open int2/int4 work.

relation to existing prs

• merged #38378 added int8_per_token_head / fp8_per_token_head
• open #39074 extends the same per-token-head path with int2_per_token_head / int4_per_token_head
• open #40128 fixes the non-divisible page-size case with an lcm-based approach

this pr is not a duplicate of #40128.

#40128 handles the page-size unification part only.
this pr also fixes two additional hybrid correctness issues that are needed for quantized per-token-head kv paths:

• preserving kv_quant_mode during hybrid spec conversion when the hybrid manager is disabled
• using gcd instead of min(...) for scheduler block size across mixed hybrid kv groups

so the scope here is: hybrid quantized per-token-head kv correctness as a whole, not just the page-size alignment step.

why this change is needed

hybrid models can mix kv specs with different page sizes and different effective block sizes.
for quantized per-token-head kv cache, this causes two kinds of problems:

• hybrid page-size unification can fail even when a valid common page size exists
• after hybrid grouping, scheduler block size can become invalid if it is derived from min(...) instead of a common divisor
• when hybrid manager is disabled, converting sliding/chunked-local specs to FullAttentionSpec can silently drop kv_quant_mode

these issues matter today for the merged int8 path from #38378, and the same generic path will also affect #39074 once int2/int4 support lands.

validation

checks run:

• pre-commit run --files vllm/v1/core/kv_cache_utils.py vllm/v1/engine/core.py tests/v1/core/test_kv_cache_utils.py
• python -m pytest tests/v1/core/test_kv_cache_utils.py -q -k 'unify_hybrid_kv_cache_specs or unify_kv_cache_spec_page_size_uses_common_multiple_for_int8_hybrid'

results:

• pre-commit: passed
• pytest: 2 passed, 49 deselected

local runtime validation:

• Qwen3.6-35B-A3B-AWQ-4bit, tp=2, kv_cache_dtype=int8_per_token_head, hybrid manager enabled
  • engine init succeeded
  • generation succeeded
• on a local branch that also included Gemma4 core support and the open int2/int4 work, the same generic fix was also validated on hybrid Gemma4/Qwen int4 paths

impact

this change is generic and model-agnostic.
it does not add model-specific branching.
it only adjusts hybrid kv spec handling so quantized per-token-head kv modes can keep using the intended hybrid manager path.

AI assistance was used for this change.

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[gemini-code-assist]

Code Review

This pull request improves the KV cache unification logic to better support hybrid configurations. Key updates include using the Least Common Multiple (LCM) to unify page sizes across different cache specs and calculating the global block size using the Greatest Common Divisor (GCD) of all cache groups. The changes also ensure that kv_quant_mode is correctly propagated during spec unification. A new test case has been added to verify these improvements, particularly for INT8 hybrid scenarios. I have no feedback to provide.

[claude]

Claude Code Review

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[lesj0610]

@heheda12345 Sorry one more. This is also small fix in hybrid KV quantized path, related to #40308 but independent.

Problem: TurboQuant layer with sliding_window was created as normal SlidingWindowSpec, because get_kv_cache_spec checked sliding_window before turboquant path. After that, hybrid unification converts it through normal FullAttentionSpec route, and TQ page/slot size info is lost.

This PR adds TQSlidingWindowSpec that keeps TQ-aware page size, makes TurboQuant sliding-window attention return it, and preserves it as TQFullAttentionSpec during hybrid unification. SlidingWindowManager also handles TQSlidingWindowSpec so sliding-window cap still works.

This touches kv_cache_interface.py and v1/core, so your review would help a lot. Small patch with tests included.