Call object_from_json from decoder when issubclass(_class, Serializat…

…ionMixin)

Summary: As titled. This solves a relatively common problem (which may crop up again in some recently planning changes) in which the init args of some serializable Ax object is itself a serializable Ax object

Differential Revision: D56356035

ax/storage/json_store/decoder.py

@@ -263,7 +263,14 @@ def object_from_json(
         elif isclass(_class) and issubclass(_class, SerializationMixin):
             return _class(
                 **_class.deserialize_init_args(
-                    args=object_json,
+                    args={
+                        key: object_from_json(
+                            object_json=value,
+                            decoder_registry=decoder_registry,
+                            class_decoder_registry=class_decoder_registry,
+                        )
+                        for key, value in object_json.items()
+                    },
                     decoder_registry=decoder_registry,
                     class_decoder_registry=class_decoder_registry,
                 )