[Model] Remove transformers attention porting in VITs

[Isotr0py]

• In [Bugfix][VLM] Add fallback to SDPA for ViT model running on CPU backend #8061, we added fallback to transformers SDPA ViT attention implementation for CPU backend.
• However, since CPU backend has supported tensor parallel in [Hardware] [Intel] Enable Multiprocessing and tensor parallel in CPU backend and update documentation  #6125, it isn't compatible with CPU TP.
• Moreover, since we have planned to consolidate ViT attention backend, the separated SDPA and xformers implementation will increase the difficulty of consolidation and maintenance.
• Therefore, we should remove the porting and consolidate SDPA implementation to parallelized ViT attention.

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can do one of these:

• Add ready label to the PR
• Enable auto-merge.

🚀

[DarkLight1337]

PTAL at the test failure.

[DarkLight1337]

Looks good now, thanks!