code: def master(self, task_type=None, task_id=None, rpc_layer=None):
      return ""
  
imports: from tensorflow.python.client import session
  from tensorflow.python.distribute.cluster_resolver import cluster_resolver
  from tensorflow.python.eager import context
  from tensorflow.python.framework import config
  from tensorflow.python.framework import test_util
  from tensorflow.python.platform import test
  from tensorflow.python.training import server_lib
label_desc: Use image node_id 2 for calling the MockBaseClusterResolver obj's underlying member method code with example usage: obj.master(task_type, task_id, rpc_layer) and returns: str