[Bugfix] Add missing VIT backend dispatch on CPU

[bigPYJ1151]

Purpose

• Add missing VIT backend dispatch on CPU to fix failed tests

Test Plan

CI tests

Test Result

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• 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.
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[gemini-code-assist]

Code Review

This pull request correctly fixes a bug in ViT attention on CPU by adding the TORCH_SDPA_VLLM_V1 backend to the dispatch logic. The change is straightforward and resolves the issue. I have one suggestion to improve the code's readability and maintainability.

[gemini-code-assist]

For better readability and consistency, consider using the in operator to check for membership in a collection. This is also how a similar check is performed in the __init__ method of this class. Using in makes the code more concise and easier to maintain, especially if more backends are added to this condition in the future.

        elif self.attn_backend in (_Backend.TORCH_SDPA, _Backend.TORCH_SDPA_VLLM_V1):