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

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

vllm/platforms/hpu.py

@@ -1,11 +1,15 @@
 import torch
 
-from .interface import Platform, PlatformEnum
+from .interface import Platform, PlatformEnum, _Backend
 
 
 class HpuPlatform(Platform):
     _enum = PlatformEnum.HPU
 
+    @classmethod
+    def get_default_attn_backend(cls, selected_backend: _Backend) -> _Backend:
+        return _Backend.HPU_ATTN
+
     @staticmethod
     def inference_mode():
         return torch.no_grad()