Disable FlashInfer sampler by default

[mgoin]

Purpose

There have been increasing reports of correctness issues or IMA with FlashInfer's top-p & top-k sampling kernel (see #26480 (comment)). For instance, it seems it can generates the same output even when the temperature is quite high (even though the seed is not set). vLLM generates different results (expectedly) once the kernel is disabled.

Since flashinfer-python is a default dep of vLLM CUDA now, many more users would be using this kernel by default. Let us disable it by default for now so users can opt-in

Test Plan

Test Result

---

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request correctly disables the FlashInfer sampler by default, requiring users to opt-in by setting VLLM_USE_FLASHINFER_SAMPLER=1. The change from envs.VLLM_USE_FLASHINFER_SAMPLER is not False to a direct boolean check simplifies the logic and makes the default behavior consistent. The corresponding log message is also appropriately changed from a warning to a debug message. The implementation is sound and improves code clarity.

[yzh119]

Hi @mgoin can we get a concrete reproducing example (or a link to the issues) for reproducing the correctness/IMA issues you mentioned? It will be very helpful for us to improve the stability.