[distributed][rl] remove nccl cumem env var override

[youkaichao]

Purpose

The env var override was initially designed to mitigate NCCL's memory overhead occuring in NCCL 2.19. Reports from NCCL team show that this env var needs to be enabled by default to enable many optimizations. And I have confirmed in NVIDIA/nccl#1234 that NCCL 2.22.3 resolved the memory overhead issue.

Given the above fact, we should remove the env var override to unleash NCCL's full potential.

One caveat is for RL frameworks, they need to be consistent with vllm w.r.t. the env var value. This can be done by checking the vllm's version.

cc @hijkzzz @vermouth1992

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 removes a workaround for an old NCCL bug by deleting the code that overrides the NCCL_CUMEM_ENABLE environment variable. This is a positive change as it allows vLLM to benefit from newer NCCL optimizations. The core logic change in vllm/env_override.py is sound. I have one suggestion for the documentation update in docs/usage/troubleshooting.md to improve clarity and prevent potential user confusion, which could lead to system hangs.

[22quinn]

Looks good, thanks for cleaning it up!

[houseroad]

Looks good.