code: def testMergedClusterResolverMaster(self):
      cluster_spec_1 = server_lib.ClusterSpec(
          {"ps": ["ps0:2222", "ps1:2222"]}
      )
      cluster_spec_2 = server_lib.ClusterSpec(
          {"worker": ["worker0:2222", "worker1:2222", "worker2:2222"]}
      )
      cluster_resolver_1 = cluster_resolver.SimpleClusterResolver(
          cluster_spec_1
      )
      cluster_resolver_2 = cluster_resolver.SimpleClusterResolver(
          cluster_spec_2
      )
  
      union_cluster = cluster_resolver.UnionClusterResolver(
          cluster_resolver_1, cluster_resolver_2
      )
  
      unspecified_master = union_cluster.master()
      self.assertEqual(unspecified_master, "")
  
      specified_master = union_cluster.master("worker", 1)
      self.assertEqual(specified_master, "worker1:2222")
  
      rpc_master = union_cluster.master("worker", 1, rpc_layer="grpc")
      self.assertEqual(rpc_master, "grpc://worker1:2222")
  
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 10 for calling the UnionClusterResolverTest obj's underlying member method code with example usage: obj.testMergedClusterResolverMaster() without return types