[Bugfix][TurboQuant] Add KV quant mode for turboquant

[skavulya]

Purpose

Adds KV quant mode for turboquant KV cache. This fix prevents the KV cache dtype string being set to auto which causes the following failure in TurboQuant observed in #47609 #48177 #48907

vllm serve Qwen/Qwen3-30B-A3B-Thinking-2507  --kv-cache-dtype turboquant_4bit_nc

(EngineCore pid=21869)   File "/tmp/vllm/vllm/v1/worker/gpu_model_runner.py", line 6499, in profile_cudagraph_memory
(EngineCore pid=21869)     self._init_minimal_kv_cache_for_profiling()
(EngineCore pid=21869)   File "/tmp/vllm/vllm/v1/worker/gpu_model_runner.py", line 6383, in _init_minimal_kv_cache_for_profiling
(EngineCore pid=21869)     self.initialize_kv_cache(minimal_config, is_profiling=True)
(EngineCore pid=21869)   File "/tmp/vllm/vllm/v1/worker/gpu_model_runner.py", line 7439, in initialize_kv_cache
(EngineCore pid=21869)     kv_caches = self.initialize_kv_cache_tensors(
(EngineCore pid=21869)                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(EngineCore pid=21869)   File "/tmp/vllm/vllm/v1/worker/gpu_model_runner.py", line 7356, in initialize_kv_cache_tensors
(EngineCore pid=21869)     kv_caches = self._reshape_kv_cache_tensors(
(EngineCore pid=21869)                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(EngineCore pid=21869)   File "/tmp/vllm/vllm/v1/worker/gpu_model_runner.py", line 7201, in _reshape_kv_cache_tensors
(EngineCore pid=21869)     kv_cache_shape = attn_backend.get_kv_cache_shape(
(EngineCore pid=21869)                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(EngineCore pid=21869)   File "/tmp/vllm/vllm/v1/attention/backends/turboquant_attn.py", line 161, in get_kv_cache_shape
(EngineCore pid=21869)     tq_config = TurboQuantConfig.from_cache_dtype(cache_dtype_str, head_size)
(EngineCore pid=21869)                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(EngineCore pid=21869)   File "/tmp/vllm/vllm/model_executor/layers/quantization/turboquant/config.py", line 222, in from_cache_dtype
(EngineCore pid=21869)     raise ValueError(
(EngineCore pid=21869) ValueError: Unknown TurboQuant cache dtype: 'auto'. Valid presets: turboquant_k8v4, turboquant_4bit_nc, turboquant_k3v4_nc, turboquant_3bit_nc

Test Plan

Test Result

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

[claude]

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