docstr: test for nested search space
          
docstr_tok: ["test","for","nested","search","space"]
code: def test_split_index_nested(self):
      """test for nested search space"""
      nested_params_with_index = {
          "layer0": {
              "_name": "Avg_pool",
              "pooling_size": {"_index": 1, "_value": 2},
          },
          "layer1": {"_name": "Empty"},
          "layer2": {
              "_name": "Max_pool",
              "pooling_size": {"_index": 2, "_value": 3},
          },
          "layer3": {
              "_name": "Conv",
              "kernel_size": {"_index": 3, "_value": 5},
              "output_filters": {"_index": 3, "_value": 64},
          },
      }
      nested_params = {
          "layer0": {"_name": "Avg_pool", "pooling_size": 2},
          "layer1": {"_name": "Empty"},
          "layer2": {"_name": "Max_pool", "pooling_size": 3},
          "layer3": {
              "_name": "Conv",
              "kernel_size": 5,
              "output_filters": 64,
          },
      }
      params = split_index(nested_params_with_index)
      self.assertEqual(params, nested_params)
  
imports: from unittest import TestCase, main
  import nni
  from nni.utils import split_index
label_desc: Use image node_id 2 for calling the UtilsTestCase obj's underlying member method code with example usage: obj.test_split_index_nested() without return types