[CI] Parallelize Kernels MoE Test

[mgoin]

Purpose

The Kernels MoE Test takes 3 hours now, so let's at least split it in half.

https://buildkite.com/vllm/ci/builds/25087/steps/canvas?sid=01984c37-20b7-47a1-b05e-6b3f88102b93

Test Plan

CI

[github-actions]

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[gemini-code-assist]

Code Review

This pull request aims to speed up the CI by parallelizing the Kernels MoE Test. The changes in the Buildkite pipeline configuration correctly add the parallelism attribute and the necessary pytest sharding flags.

However, I've identified a critical issue with the implementation of test sharding. The Buildkite environment variable BUILDKITE_PARALLEL_JOB is 0-indexed, while the pytest-shard plugin (which is likely being used here) expects a 1-indexed --shard-id. This mismatch will cause the first parallel job to fail, preventing a significant portion of the tests from running and breaking the build. I've provided suggestions to correct this for both the newly parallelized MoE test and an existing quantization test that has the same issue.