[Bugfix] Set VLLM_ALLREDUCE_USE_SYMM_MEM default to False

[yewentao256]

Purpose

Fixes #24694

#24111 should be tested with DP case then open to default again

Test

(APIServer pid=454020) INFO:     Started server process [454020]
(APIServer pid=454020) INFO:     Waiting for application startup.
(APIServer pid=454020) INFO:     Application startup complete.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @yewentao256.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[gemini-code-assist]

Code Review

This pull request correctly disables the VLLM_ALLREDUCE_USE_SYMM_MEM feature by default by changing its environment variable's default value from True to False. This is a sensible approach to temporarily mitigate a bug as described in the PR. The changes are consistently applied, and the existing tests for this feature are correctly configured to explicitly enable it, ensuring continued test coverage. The implementation looks good.

[mgoin]

Thanks, let's get this bugfix in for now to fix release

[nvpohanh]

@ilmarkov Could you fix the issue and re-enable VLLM_ALLREDUCE_USE_SYMM_MEM by default so that we can benefit from the faster AllReduce without any env vars? Thanks!

[ilmarkov]

@nvpohanh Yes, I am working on this. The easiest solution would be disable symm mem when DP is used (i.e. all devices do only TP or PP) but I am trying to find a way to enable it for all. The problem is torch incorrectly detects overlapping devices here in case when multiple DP processes are running on the same node.