[Misc] Enable yapf for FlashInfer backend

[WoosukKwon]

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[gemini-code-assist]

Code Review

This pull request enables yapf for the FlashInfer backend, resulting in a series of code formatting changes. These modifications are purely stylistic and align the code with yapf's formatting standards, improving readability and consistency. I have reviewed the changes, and they are correct and do not introduce any functional alterations. The pull request is ready to be merged.