code: def lookup(self, key):
      self.invocation_counter_map["lookup"] += 1
      return None
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from absl.testing import parameterized
  from neural_structured_learning import configs
  from research.carls import graph_regularization
  from research.carls import neighbor_cache_client
  import numpy
  import tensorflow
  from google.protobuf import text_format
  from tensorflow.python.framework import test_util
label_desc: Use image node_id 2 for calling the MockNeighborCacheClient obj's underlying member method code with example usage: obj.lookup(key) and returns: None