Feature request: Support sleep mode

vLLM's sleep mode is not aviable for the Intel Backend. To save on Idle Power Consumption it would be great to add the XPU backend

Start by locating the existing sleep-mode implementation and the Intel/XPU backend entry points in llm-scaler. Compare how sleep mode is supported for other backends, then identify the XPU-specific work needed and verify that idle power consumption is reduced without breaking backend operation.

Written by the indexing model from the issue text.