Cache NPU device probe to avoid per-dispatch xrt-smi subprocess

[erwei-xilinx]

Summary

• get_npu_device_info() runs xrt-smi examine (~30 ms subprocess) to detect the NPU generation (npu1/npu2).
• It is reachable from the per-dispatch hot path via detect_npu_version(), so every kernel launch could spawn a subprocess just to re-answer a question whose result is constant for the process lifetime.
• Memoize it with functools.lru_cache(maxsize=1). Physical devices don't change within a process. The AMD_TRITON_NPU_TARGET cross-compile override still short-circuits in detect_npu_version() before the probe is ever called, so that path is unaffected.

Impact

Measured on NPU2/Strix, this removes a uniform ~30 ms/dispatch overhead on hot paths that reach detect_npu_version() per launch, with no behavior change (same device detection result, just computed once).

Test plan

• python -m py_compile on the changed file
• Device detection still returns the correct generation (npu2) after caching
• CI build

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[Copilot]

🟢 Ready to approve

The functional change is small and low-risk, and the only noted issue is a minor import cleanup nit.

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Pull request overview

This PR optimizes the AMD Triton NPU backend hot path by memoizing NPU device probing so xrt-smi examine is executed once per process instead of potentially once per dispatch.

Changes:

• Added functools.lru_cache(maxsize=1) to get_npu_device_info() to avoid repeated xrt-smi examine subprocess calls.
• Added an explanatory comment documenting why the probe result is safe to cache for process lifetime.
• Updated imports to include functools.

File summaries

File	Description
amd_triton_npu/backend/driver.py	Caches NPU device probe results to eliminate per-dispatch subprocess overhead.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Lite

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