[Core] Remove xformers dependency

[ywang96]

Purpose

This PR completely removes the dependency of xformers library and should be only merged after v0.11.1 release. The rationale behind removing xformers is that:

1. xformers is used for multimodal attention (MHA) but we can have alternative attention backends to replace it
2. We have xformers attention backend for decoder LM, but it's no longer used for anything
3. Having another external dependency puts extra risks on our release - a hard lesson we learned from working on upgrading pytorch 2.9.

For 1, the alternatives are:

• Compile for commom head sizes of MHA in our fork of FA so we don't need to use xformers as a fallback
• Triton

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.

[mergify]

Documentation preview: https://vllm--28287.org.readthedocs.build/en/28287/

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @ywang96.

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

[ywang96]

Going to create a new PR for this since there are quite a few conflicts.