fix bad error message when transformers is missing (#4714)

src/diffusers/utils/import_utils.py

@@ -567,7 +567,7 @@ class DummyObject(type):
     """
 
     def __getattr__(cls, key):
-        if key.startswith("_") and key != "_load_connected_pipes":
+        if key.startswith("_") and key not in ["_load_connected_pipes", "_is_onnx"]:
             return super().__getattr__(cls, key)
         requires_backends(cls, cls._backends)