fix `_optional_components` in `StableCascadeCombinedPipeline` (#7894)

* fix

* up

src/diffusers/pipelines/stable_cascade/pipeline_stable_cascade_combined.py

@@ -71,6 +71,7 @@ class StableCascadeCombinedPipeline(DiffusionPipeline):
     """
 
     _load_connected_pipes = True
+    _optional_components = ["prior_feature_extractor", "prior_image_encoder"]
 
     def __init__(
         self,

tests/pipelines/stable_cascade/test_stable_cascade_combined.py

@@ -153,6 +153,8 @@ class StableCascadeCombinedPipelineFastTests(PipelineTesterMixin, unittest.TestC
             "prior_tokenizer": prior_tokenizer,
             "prior_prior": prior,
             "prior_scheduler": scheduler,
+            "prior_feature_extractor": None,
+            "prior_image_encoder": None,
         }
 
         return components