perf: route low-token qwen3 decode through optimized nz quantmatmul.

[maojunx99]

Description

• Add a dedicated graph variant for optimized W8A8 BF16 decode.
• Route eligible QKV, gate/up, and down projections with up to 16 tokens.
• Preserve the original graph and eager paths as fallbacks for unsupported shapes.
• Update xllm-ops and xllm-atb-layers

Change Type

• Bug fix
• New feature
• Performance improvement
• Refactor
• Documentation
• Test
• Build or CI

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Self Review

• I have self-reviewed the code according to .agents/skills/code-review/references/custom-code-style.md, especially code written or assisted by AI.
• I have rebased this PR onto the latest main branch.

Build and Test Coverage

• Tests have been added or updated as needed.
• CUDA: python setup.py build test has passed on a CUDA machine.
• NPU: python setup.py build test has passed on an NPU machine.
• MLU: python setup.py build test has passed on an MLU machine.