[Misc] Add generated git commit hash as vllm.__commit__

[mgoin]

This PR embeds the current git commit hash of the source code into the installed library.

1. The hash itself is defined in the __commit_ variable in vllm/version.py alongside __version__, with an initial dummy string "COMMIT_HASH_PLACEHOLDER"
2. It writes the commit id to the untracked vllm/commit_id.py file instead of modifying version.py, mainly to prevent somebody from accidentally committing their commit IDs, and to keep setup.py from polluting developers' git status.

After you run pip install -e ., you should see something like:

>>> import vllm
>>> print(vllm.__version__)
0.5.1
>>> print(vllm.__commit__)
eb7557fbb40553ff4e4f7cd8dedd4297d1bb2111

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
Just a reminder: PRs would not trigger full CI run by default. Instead, it would only trigger fastcheck CI to run, which consists only a small and essential subset of tests to quickly catch small errors.

Full CI run is still required to merge this PR so please make sure that you run full CI before merging or if you need more test signals.

To run full CI, you can do one of these:

• Add ready label to the PR
• Comment /ready on the PR
• Enable auto-merge.

🚀

[comaniac]

LGTM!

[youkaichao]

@mgoin this actually does not work in ci. all ci jobs have this warning now. see https://buildkite.com/vllm/ci-aws/builds/4840#0190a94d-30c0-4ef4-96fa-55d943ac116b/178-326 for example.

the test you add, is not run in ci I think.