[Misc] auto_tune: kill specific vllm process

[karan]

Purpose

The pkill is too broad and can kill other process with vllm in their path (for eg an unrelated script).

Test Plan

Tested locally.

Test Result

Works as expected.

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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[gemini-code-assist]

Code Review

This pull request aims to make the pkill command more specific to avoid accidentally killing unrelated processes. The change from pkill -if vllm to pkill -if "vllm serve" is a good step in that direction. However, this approach is still brittle as it relies on matching the process command line string. I've suggested a more robust alternative that identifies and kills the server process by the port it's listening on (8004). This ensures that any server process using that port is terminated, regardless of how it was started, preventing potential script failures. Additionally, the warning in benchmarks/auto_tune/README.md about not having vllm in the script path is now outdated with your change and could be removed or updated.

[karan]

@yaochengji Mind reviewing this? I messed up my git history in the previous branch.

[yaochengji]

LGTM!