update get_split_k to fix a performance regression on FA decode

[scxiao]

What does this PR do?

Fixes a performance regress for FA decode

Before submitting

• [x] Did you have fun?
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• [ ] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
  • [x] N/A
• [ ] Did you make sure to update the docs?
  • [x] N/A
• [ ] Did you write any new necessary tests?
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• [ ] Did you update the changelog? (if needed)
  • [x] N/A

PR review

Anyone in the community is free to review the PR once the tests have passed. If we didn't discuss your PR in Github issues there's a high chance it will not be merged.