update save method in pipeline

src/tensorneat/common/stateful_class.py

@@ -33,11 +33,16 @@ class StatefulBaseClass:
 
         return state
 
-    def show_config(self):
+    def show_config(self, registered_objects=None):
+        if registered_objects is None:  # root call
+            registered_objects = []
+
         config = {}
         for key, value in self.__dict__.items():
-            if isinstance(value, StatefulBaseClass):
-                config[str(key)] = value.show_config()
+            if isinstance(value, StatefulBaseClass) and value not in registered_objects:
+                registered_objects.append(value)
+                config[str(key)] = value.show_config(registered_objects)
+
             else:
                 config[str(key)] = str(value)
         return config