Fix serialization / deserialization. (#20406)

- Serialization was not taking the registered name and package from the registry.
- Deserialization was selecting symbols by postfix as a fallback.

keras/src/saving/saving_lib_test.py

@@ -367,7 +367,7 @@ def test_saved_module_paths_and_class_names(self):
         )
         self.assertEqual(
             config_dict["compile_config"]["loss"]["config"],
-            "my_mean_squared_error",
+            "my_custom_package>my_mean_squared_error",
         )
 
     @pytest.mark.requires_trainable_backend

keras/src/saving/serialization_lib.py

@@ -366,7 +366,7 @@ def _get_class_or_fn_config(obj):
     """Return the object's config depending on its type."""
     # Functions / lambdas:
     if isinstance(obj, types.FunctionType):
-        return obj.__name__
+        return object_registration.get_registered_name(obj)
     # All classes:
     if hasattr(obj, "get_config"):
         config = obj.get_config()
@@ -781,15 +781,6 @@ def _retrieve_class_or_fn(
                 if obj is not None:
                     return obj
 
-            # Retrieval of registered custom function in a package
-            filtered_dict = {
-                k: v
-                for k, v in custom_objects.items()
-                if k.endswith(full_config["config"])
-            }
-            if filtered_dict:
-                return next(iter(filtered_dict.values()))
-
         # Otherwise, attempt to retrieve the class object given the `module`
         # and `class_name`. Import the module, find the class.
         try: