[Bug] fix import and unit test

[jmkuebler]

Purpose

#24845 introduced a wrong import which results in errors of pytest tests/v1/attention/test_attention_splitting.py.

Test Plan

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request correctly fixes a broken import in tests/v1/attention/test_attention_splitting.py. The function create_ubatch_slices was being imported from vllm.v1.worker.ubatch_utils, but it is defined in vllm.v1.worker.ubatch_splitting. The change updates the import path to the correct location, which resolves the test failure mentioned in the description. The change is accurate and I have no further feedback.

[jmkuebler]

@LucasWilkinson #24845 breaks the unit tests, because of a wrong import. This fixes it.