fix: tiny fix cli (#12744)

python/sglang/multimodal_gen/runtime/server_args.py

@@ -777,6 +777,13 @@ class ServerArgs:
                 )
             self.sp_degree = self.ulysses_degree = self.ring_degree = 1
 
+        if (
+            self.ring_degree is not None
+            and self.ring_degree > 1
+            and self.attention_backend != "fa3"
+        ):
+            raise ValueError("Ring Attention is only supported for fa3 backend for now")
+
         if self.sp_degree == -1:
             # assume we leave all remaining gpus to sp
             num_gpus_per_group = self.dp_size * self.tp_size