add code reference to intel device key

Signed-off-by: youkaichao <[email protected]>

vllm/platforms/xpu.py

@@ -19,7 +19,8 @@ class XPUPlatform(Platform):
     device_name: str = "xpu"
     device_type: str = "xpu"
     dispatch_key: str = "XPU"
-    # does XPU have a ray device key?
+    # Intel XPU's device key is "GPU" for Ray.
+    # see https://github.com/ray-project/ray/blob/6a5eb5865eeb9ccf058a79b44f107e327e360673/python/ray/_private/accelerators/intel_gpu.py#L20 # noqa: E501
     ray_device_key: str = "GPU"
 
     @classmethod