code: def test_model_to_json(self):
      network = _DummyUnivariateRankingNetwork(
          context_feature_columns=self.context_feature_columns,
          example_feature_columns=self.example_feature_columns,
          name="dummy_univariate_ranking_network",
      )
      ranker = model_lib.create_keras_model(
          network=network,
          loss=self.loss,
          metrics=self.metrics,
          optimizer=self.optimizer,
          size_feature_name="example_list_size",
      )
  
      json_config = ranker.to_json()
      custom_objects = {
          "GenerateMask": feature.GenerateMask,
          "_DummyUnivariateRankingNetwork": _DummyUnivariateRankingNetwork,
      }
      restored_ranker = tf.keras.models.model_from_json(
          json_config, custom_objects=custom_objects
      )
      self.assertAllEqual(
          restored_ranker(self.features), ranker(self.features)
      )
  
imports: from absl.testing import parameterized
  import tensorflow
  from google.protobuf import text_format
  from tensorflow_ranking.python import data
  from tensorflow_ranking.python.keras import feature
  from tensorflow_ranking.python.keras import losses
  from tensorflow_ranking.python.keras import metrics
  from tensorflow_ranking.python.keras import model
  from tensorflow_ranking.python.keras import network
  from tensorflow_serving.apis import input_pb2
label_desc: Use image node_id 3 for calling the FunctionalRankingModelTest obj's underlying member method code with example usage: obj.test_model_to_json() without return types