[Bugfix] Detect CUDA via the driver API when NVML is unavailable

[RohitNagraj]

Purpose

Fix CUDA platform detection in environments where NVML is absent or non-functional while the CUDA driver API works.

cuda_platform_plugin() currently detects CUDA exclusively through pynvml, with a Jetson file-marker check as the only non-NVML fallback. There are environments where nvmlInit() fails ("Driver Not Loaded") while libcuda is fully functional — a reproducible case is certain WSL2 setups where the GPU driver injects working libcuda stubs into /usr/lib/wsl/lib but NVML cannot communicate with the driver (nvidia-smi fails the same way). In those environments vLLM exits with:

RuntimeError: Failed to infer device type

even though CUDA is fully usable — NonNvmlCudaPlatform in vllm/platforms/cuda.py already supports serving without NVML once the platform is selected; it's only the detection step that is NVML-only.

This PR adds a driver-API fallback probe (cuInit + cuDeviceGetCount) in the NVML-failure branch, mirroring the existing Jetson special case. Design notes:

• The probe runs in a short-lived subprocess, so CUDA is never initialized in the parent process before workers may fork (the reason detection avoids the CUDA runtime in the first place; see also [Bugfix] Detect driver-level CUDA init before fork #44252 / [Bug] Avoid Triton 3.8 nvidia cuInit poisoning forked CUDA init #47078 for the fork-poisoning class of issues).
• It only executes on the existing NVML-failure path — zero behavior/overhead change for machines where NVML works, for CPU-only machines (libcuda.so.1 missing → probe returns False), and for cpu builds (existing vllm_version_matches_substr("cpu") guard applied).
• The related import vllm circular-import crash on WSL is not addressed here — it is already covered by [Bugfix] Fix WSL circular import from pin_memory warning_once #48444; this change is complementary (after [Bugfix] Fix WSL circular import from pin_memory warning_once #48444 fixes the import, platform detection is the next failure on NVML-less systems).

Why this is not duplicating an existing PR: searched open PRs/issues for cuda_platform_plugin, "NVML platform detection", "Failed to infer device type", and WSL detection fixes. #48444 fixes the WSL pin-memory circular import (different bug, see above); #38434 / #41585 are ROCm detection; #44252 / #47078 address CUDA-init-before-fork (this PR deliberately avoids introducing that problem via the subprocess probe). No open PR touches NVML-less CUDA detection.

AI assistance disclosure: this change was developed with AI assistance (Claude). The submitter has reviewed every changed line and run the validation below.

Test Plan

1. Environment where the bug reproduces (WSL2, working libcuda, non-functional NVML — nvidia-smi reports "couldn't communicate with the NVIDIA driver"):

   python -c "from vllm.platforms import current_platform; print(current_platform.device_type)"

   Before: RuntimeError: Failed to infer device type. After: cuda (resolves to NonNvmlCudaPlatform).
2. Probe unit behavior, standalone:
   • On the affected WSL2 environment: probe subprocess exits 0 (usable).
   • On a machine without libcuda.so.1 (macOS/CPU-only): probe exits non-zero (not usable) — detection outcome unchanged.
3. Regression: on a standard Linux + NVML machine the NVML path succeeds first and the new code never runs.
4. Lint: pre-commit run --files vllm/platforms/__init__.py — all hooks pass (including ruff check/format and the no-new-torch.cuda-calls check).

Test Result

• Affected WSL2 environment (NVML non-functional): probe subprocess returncode 0 → platform resolves to NonNvmlCudaPlatform / cuda. With a functionally identical patch applied to a v0.25.1 venv on that machine, vllm serve Qwen/Qwen3-8B-FP8 started and completed a full OpenAI-serving benchmark grid (ISL 1k/8k × concurrency 1/2/4, 140/140 requests, CUDA graphs enabled), where unpatched vLLM cannot pass platform detection at all.
• macOS host without libcuda: probe returncode 1 → no CUDA platform (unchanged).
• pre-commit run --files vllm/platforms/__init__.py: Passed.

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