[Platform][Refactor] Extract func get_default_attn_backend to Platform (#10358)

Signed-off-by: Mengqing Cao <cmq0113@163.com>

vllm/platforms/xpu.py

@@ -1,11 +1,21 @@
 import torch
 
-from .interface import DeviceCapability, Platform, PlatformEnum
+from vllm.logger import init_logger
+
+from .interface import DeviceCapability, Platform, PlatformEnum, _Backend
+
+logger = init_logger(__name__)
 
 
 class XPUPlatform(Platform):
     _enum = PlatformEnum.XPU
 
+    @classmethod
+    def get_default_attn_backend(cls, selected_backend: _Backend) -> _Backend:
+        if selected_backend != _Backend.IPEX:
+            logger.info("Cannot use %s backend on XPU.", selected_backend)
+        return _Backend.IPEX
+
     @staticmethod
     def get_device_capability(device_id: int = 0) -> DeviceCapability:
         major, minor, *_ = torch.xpu.get_device_capability(