[4/N][torch.compile] clean up set_torch_compile_backend

[youkaichao]

after #10383 , we can store much information in the compilation config, rather than leaving them as global variables.

[github-actions]

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[WoosukKwon]

tbh I don't think I'm the right person to review this PR; I don't have enough background 😅. However, since the PR seems to be well scoped and is mostly minor refactoring, please go ahead and merge the PR if no one else can review this.