code: def testSignatureImplementationIsInvisible(self):
      with tf.Graph().as_default() as graph:
          saved_model_lib.add_signature("test", {}, {})
          self.assertEqual(graph.get_all_collection_keys(), [])
  
      handler = saved_model_lib.SavedModelHandler()
      handler.add_graph_copy(graph)
      (meta_graph,) = handler.meta_graphs
      self.assertEqual(len(meta_graph.collection_def), 0)
      self.assertEqual(len(meta_graph.signature_def), 1)
  
imports: import copy
  import os
  from absl import logging
  import tensorflow
  from tensorflow_hub import saved_model_lib
  from tensorflow.core.protobuf import meta_graph_pb2
label_desc: Use image node_id 7 for calling the SavedModelLibTest obj's underlying member method code with example usage: obj.testSignatureImplementationIsInvisible() without return types