code: def test_estimator(self, loss_key):
      pip = self._create_pipeline(loss_key)
      self.assertIsInstance(pip._estimator, tf_estimator.Estimator)
  
imports: import os
  from absl.testing import parameterized
  import tensorflow
  from tensorflow import estimator
  import tensorflow_ranking
  from google.protobuf import text_format
  from tensorflow_ranking.extension import pipeline
  from tensorflow_serving.apis import input_pb2
label_desc: Use image node_id 5 for calling the RankingPipelineTest obj's underlying member method code with example usage: obj.test_estimator(loss_key) without return types