Experimental attention backend in helion

[bringlein]

Purpose

very experimental and draft PR so far

Test Plan

VLLM_ATTENTION_BAKCEND=EXPERIMENTAL_HELION_ATTN vllm serve meta-llama/Llama-3.1-8B-Instruct 

Test Result

t.b.a.

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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.

[mergify[bot]]

This pull request has merge conflicts that must be resolved before it can be merged. Please rebase the PR, @bringlein.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[mergify[bot]]

Documentation preview: https://vllm--27293.org.readthedocs.build/en/27293/

[mergify[bot]]

Documentation preview: https://vllm--27293.org.readthedocs.build/en/27293/