[Platform] platform agnostic for EngineArgs initialization

[wangxiyuan]

EngineArgs initialization runs before plugin loading, so it should be platform agnostic.

Once platform plugin is supported, calling platform related code during EngineArgs initialization will lead error.

This PR clean up the platform specified code during EngineArgs initialization step.

[github-actions]

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[yansh97]

I think there should be a friendly way to quickly look up the default values of parameters. It's a bit unintuitive to initialize all platform-related parameters to None right now.

[youkaichao]

I think there should be a friendly way to quickly look up the default values of parameters

when you run vllm, there should be one line of logging showing all the values (which are default values). It's a bit unintuitive, but we have no choice since we need to support multiple platforms.