[UX] Fallback to native implementation when flashinfer sampler failed to compile

[Isotr0py]

Purpose

• After [UX] Add FlashInfer as default CUDA dependency #26443, flashinfer became a default dependency, but flashinfer-cubin and flashinfer-jit-cache are not included. With cubin and jit-cache missing, flashinfer sampler can fail to compile during runtime due to various reasons:

(EngineCore_DP0 pid=7122) RuntimeError: Ninja build failed. Ninja output:
(EngineCore_DP0 pid=7122) ninja: Entering directory `/root/.cache/flashinfer/75/cached_ops'
...
(EngineCore_DP0 pid=7122) /usr/bin/ld: cannot find -lcuda: No such file or directory

(EngineCore_DP0 pid=815718) RuntimeError: Ninja build failed. Ninja output:
(EngineCore_DP0 pid=815718) ninja: Entering directory `/home/mozf/.cache/flashinfer/86/cached_ops'
(EngineCore_DP0 pid=815718) ninja: error: '/home/mozf/develop-projects/vllm/.venv/lib/python3.12/site-packages/flashinfer/data/csrc/flashinfer_sampling_ops.cu', needed by 'sampling/flashinfer_sampling_ops.cuda.o', missing and no known rule to make it

• This PR allows sampler falled back to native implementation when flashinfer ones failed to compile, and logging a message to suggest user to install flashinfer-cubin and flashinfer-jit-cache manually.

Test Plan

Test Result

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[gemini-code-assist]

Code Review

This pull request introduces a fallback mechanism for the flashinfer sampler. If flashinfer fails to compile its kernels at runtime, vLLM will now gracefully fall back to its native PyTorch-based sampler implementation and log a helpful warning message. This improves the user experience for those who have flashinfer installed but are missing its pre-compiled kernels. My review includes a critical suggestion to also handle AttributeError in the exception block to prevent a potential fatal error during module import if an older version of flashinfer is used.

[chatgpt-codex-connector]

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[mgoin]

We disabled flashinfer sampler by default last night #26859, so I think this change with make it such that users will see this warning for now reason since it is immediately attempting to compile on import. Could we make this only trigger when we are requesting the flashinfer sampler?

[Isotr0py]

Could we make this only trigger when we are requesting the flashinfer sampler?

Done in e0b95c2