[Hardware][NIXL] set default kv buffer type for different platform (#36438)

Signed-off-by: zhenwei-intel <zhenwei.liu@intel.com>
Co-authored-by: Kunshang Ji <kunshang.ji@intel.com>

vllm/config/kv_transfer.py

@@ -24,9 +24,9 @@ class KVTransferConfig:
     engine_id: str | None = None
     """The engine id for KV transfers."""
 
-    kv_buffer_device: str = "cuda"
-    """The device used by kv connector to buffer the KV cache. Choices are 
-    'cuda' and 'cpu'."""
+    kv_buffer_device: str | None = None
+    """The device used by kv connector to buffer the KV cache. Choices are
+    'cuda','cpu' and 'xpu'."""
 
     kv_buffer_size: float = 1e9
     """The buffer size for TorchDistributedConnector. Measured in number of
@@ -100,6 +100,11 @@ class KVTransferConfig:
                 f"is set, supported roles are {get_args(KVRole)}"
             )
 
+        if self.kv_buffer_device is None:
+            from vllm.platforms import current_platform
+
+            self.kv_buffer_device = current_platform.device_type
+
     @property
     def is_kv_transfer_instance(self) -> bool:
         return self.kv_connector is not None and self.kv_role in get_args(KVRole)