[Feature]: config init check sleep mode support oot platforms

[celestialli]

🚀 The feature, motivation and pitch

As it is coded in /vllm/config.py

        if self.enable_sleep_mode and not current_platform.is_cuda():
            raise ValueError("Sleep mode is only supported on CUDA devices.")

While there are oot platforms, which will support sleep mode (Ascend NPU has already done this in v0.7.3 branch)
The above code makes it hard to support sleep mode in oot platforms.

A possible solution is to add a method to base class Platform: is_sleep_mode_available(), oot Platform classes can override this method to indicate whether the platform supports sleep mode.

Alternatives

No response

Additional context

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