[CPU] Support head_size 512 in cpu_attn

[bigPYJ1151]

Purpose

Add 512 head_size support for in-coming models.

Test Plan

CI tests

Test Result

---

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.

[mergify]

Documentation preview: https://vllm--38676.org.readthedocs.build/en/38676/

[gemini-code-assist]

Code Review

This pull request adds support for a head size of 512 to the CPU attention backend. The changes include updating the dispatch generation script, documentation, unit tests, and the backend's supported head sizes list. I have no feedback to provide.