[Misc] Add numpy implementation of compute_slot_mapping

[Yard1]

For prefill, the python implementation of compute_slot_mapping is inefficient as we loop over large lists. This PR adds a numpy implementation of the same operation, letting us leverage numpy vectorized instructions for up to 3x speedup with large lists. If the number of slots is small, we still use the python implementation as the overheads from creating numpy arrays are too great and actually cause a slowdown.

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[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 run fastcheck CI which consists a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of default ones by unblocking the steps in your fast-check build on Buildkite UI.

Once the PR is approved and ready to go, please make sure to run full CI as it is required to merge (or just use auto-merge).

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

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

🚀

[Yard1]

/ready

[youkaichao]

List can directly extend array?

[Yard1]

Yep!

[comaniac]

LGTM!
I manually trigger more tests for verification.