text: Title: if _name is not defined in HPO space, the experiment will not stop.
  Body: **Describe the issue**:
  When testing nested sub-search-space in HPO, if _name is not defined, the experiment will not stop.
  
  
  **Environment**:
  - NNI version: 2.10
  - Training service (local|remote|pai|aml|etc): local
  - Client OS: linux
  - Server OS (for remote mode only):
  - Python version: 3.7
  - PyTorch/TensorFlow version: 
  - Is conda/virtualenv/venv used?: conda
  - Is running in Docker?: no
  
  
  **Configuration**:
   - Experiment config (remember to remove secrets!):
   - Search space:
  
  
  **Log message**:
   - nnimanager.log:
   - dispatcher.log: **RuntimeError: '_name' key is not found in this nested search space.**
   - nnictl stdout and stderr:
   
  <!--
  Where can you find the log files:
  LOG: https://github.com/microsoft/nni/blob/master/docs/en_US/Tutorial/HowToDebug.md#experiment-root-director
  STDOUT/STDERR: https://nni.readthedocs.io/en/stable/reference/nnictl.html#nnictl-log-stdout
  -->
  
  
  **How to reproduce it?**:
  command: `python test_hpo_space.py --space test.yaml`
  test_hpo_space.py:
  ```
  import nni
  import numpy as np
  import torch
  import os
  import logging
  import random
  import time
  import argparse
  import json
  import yaml
  
  nni.silence_stdout()
  from nni.experiment import Experiment
  
  
  def run_trial():
      param = nni.get_next_parameter()
      logging.info(f"param: {param}")
      # time.sleep(1)
      nni.report_final_result(random.random())
  
  
  def main(space: dict):
      experiment = Experiment("local")
      experiment.config.trial_command = f"python test_hpo_space.py run_trial --space 123"
  
      experiment.config.experiment_name = "HPO"
      experiment.config.trial_code_directory = os.getcwd()
      experiment.config.search_space = space
      experiment.config.tuner.name = "Evolution"
      experiment.config.tuner.class_args["optimize_mode"] = "maximize"
      experiment.config.tuner.class_args["population_size"] = 60
      experiment.config.max_trial_number = 60
      experiment.config.trial_concurrency = 10
  
      experiment.start(18189, debug=True, run_mode=nni.experiment.RunMode.Background)
  
      try:
          experiment._wait_completion()
      except KeyboardInterrupt:
          logging.warning("KeyboardInterrupt detected")
      finally:
          experiment.stop()
  
  
  if __name__ == "__main__":
      parser = argparse.ArgumentParser()
      parser.add_argument("--space", type=str, required=True)
      args, extra_args = parser.parse_known_args()
      if "run_trial" in extra_args:
          run_trial()
      else:
          space_file = args.space
          try:
              space = json.load(open(space_file))
          except Exception:
              with open(space_file, "r", encoding="utf-8") as f:
                  space = yaml.safe_load(f)
          main(space)
  ```
  
  test.yaml:
  ```
  layer0:
    _type: choice
    _value:
      - Empty
      - kernel_size:
          _type: choice
          _value: [1, 2, 3, 5]
      - _name: Max_pool
        pooling_size:
          _type: choice
          _value: [2, 3, 5]
      - _name: Avg_pool
        pooling_size:
          _type: choice
          _value: [2, 3, 5]
  ```
  