[Misc] Set default backend to SDPA for get_vit_attn_backend (#12235)

Signed-off-by: wangxiyuan <wangxiyuan1007@gmail.com>

vllm/model_executor/models/vision.py

@@ -82,23 +82,25 @@ def get_vit_attn_backend(support_fa: bool = False) -> _Backend:
         if backend_by_env_var is not None:
             selected_backend = backend_name_to_enum(backend_by_env_var)
     if selected_backend is None:
-        # For Volta and Turing GPUs, use xformers instead.
-        device_available = current_platform.has_device_capability(80)
-        if device_available and support_fa:
-            from transformers.utils import is_flash_attn_2_available
-            if is_flash_attn_2_available():
-                selected_backend = _Backend.FLASH_ATTN
+        if current_platform.is_cuda():
+            device_available = current_platform.has_device_capability(80)
+            if device_available and support_fa:
+                from transformers.utils import is_flash_attn_2_available
+                if is_flash_attn_2_available():
+                    selected_backend = _Backend.FLASH_ATTN
+                else:
+                    logger.warning_once(
+                        "Current `vllm-flash-attn` has a bug inside vision "
+                        "module, so we use xformers backend instead. You can "
+                        "run `pip install flash-attn` to use flash-attention "
+                        "backend.")
+                    selected_backend = _Backend.XFORMERS
             else:
-                logger.warning_once(
-                    "Current `vllm-flash-attn` has a bug inside vision module, "
-                    "so we use xformers backend instead. You can run "
-                    "`pip install flash-attn` to use flash-attention backend.")
+                # For Volta and Turing GPUs, use xformers instead.
                 selected_backend = _Backend.XFORMERS
-        elif current_platform.is_cpu() or current_platform.is_rocm():
-            # ROCM doesn't support xformers
-            selected_backend = _Backend.TORCH_SDPA
         else:
-            selected_backend = _Backend.XFORMERS
+            # Default to torch SDPA for other non-GPU platforms.
+            selected_backend = _Backend.TORCH_SDPA
     return selected_backend