Reduce TurboQuant KV memory loss by deduplicating decode scratch buffers

[lesj0610]

Before this PR, each TurboQuant attention layer kept three decode scratch buffers (_tq_mid_o_buf, _tq_output_buf, _tq_lse_buf) as persistent register_buffer. These are temporary scratch only, not real state. But they stayed allocated per layer, so KV cache memory was wasted proportional to the number of attention layers.

This PR removes those per-layer buffers. Each layer now calls reserve_turboquant_decode_workspace() at init, and all layers share three workspace tensors from WorkspaceManager at decode time.

I ran the duplicate check before opening:

gh pr list --repo vllm-project/vllm --state open --search "turboquant decode"

The closest result is vllm-project#40655. That PR puts one shared buffer on the Attention class. This PR uses the existing v1 workspace lifecycle instead (reserve before warmup, lock, then acquire at runtime). Shared state does not go on the Attention class, so the pipeline parallelism concern raised in vllm-project#40655 is addressed differently here.

If WorkspaceManager is not initialized, decode falls back to the previous lazy per-layer buffer reuse path.

KV cache memory — Qwen3-8B, TP=2, RTX 3090

preset	branch	KV mem	tokens
turboquant_k8v4	origin/main	12.0 GiB	387,248
turboquant_k8v4	this PR	14.02 GiB	452,224
turboquant_4bit_nc	origin/main	12.0 GiB	508,512
turboquant_4bit_nc	this PR	14.02 GiB	593,824

For turboquant_4bit_nc, short chat also returned 서울 on both branches.

Tests:

.venv/bin/python -m pytest tests/quantization/test_turboquant.py \
  -k 'init_turboquant_does_not_create_per_layer_decode_buffers or \
      workspace_reservation_uses_max_not_sum_for_heterogeneous_heads or \
      workspace_acquire_after_lock_no_growth or \
      decode_uses_layer_fallback_when_workspace_unavailable' -q

pre-commit run ruff-check --files \
  tests/quantization/test_turboquant.py \
  vllm/model_executor/layers/attention/attention.py \
  vllm/v1/attention/backends/turboquant_attn.py

Both passed.

AI assistance was used for draft and local editing support.

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