[hardware][rocm] allow rocm to override default env var

[youkaichao]

fixes #7791

[github-actions]

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[hongxiayang]

Thanks for the fix. @youkaichao

[clintg6]

This does not work. The if statement will never execute here because it is initially None not fork. All that's needed is

os.environ["VLLM_WORKER_MULTIPROC_METHOD"] = "spawn"
logger.warning("VLLM_WORKER_MULTIPROC_METHOD is overridden to `spawn`.")

[youkaichao]

how about now?

[clintg6]

Tested successfully runs and handles inference requests. Fixes issue in #7791.