Add an assertion in topk_voting_classifier's get_pipeline implementation

to verify that the result is a Pipeline before calling export_to_sklearn_pipeline on it.

This assertion should be removed once Operator supports get_pipeline, as per IBM#544

lale/lib/lale/topk_voting_classifier.py

@@ -110,6 +110,7 @@ def get_pipeline(self, pipeline_name=None, astype="lale"):
         if result is None or astype == "lale":
             return result
         assert astype == "sklearn", astype
+        assert isinstance(result, lale.operators.BasePipeline)
         return result.export_to_sklearn_pipeline()