Fix example issue (#4456)

examples/experiment_config.yml

-authorName:
-experimentName:
-trialConcurrency:
-maxExecDuration:
-maxTrialNum:
-#choice: local, remote
-trainingServicePlatform:
-searchSpacePath:
-#choice: true, false
-useAnnotation:
-tuner:
-  #choice: TPE, Random, Anneal, Evolution, BatchTuner, MetisTuner, GPTuner
-  builtinTunerName:
-  classArgs:
-    #choice: maximize, minimize
-    optimize_mode:
-assessor:
-  #choice: Medianstop, Curvefitting
-  builtinAssessorName:
-  classArgs:
-    #choice: maximize, minimize
-    optimize_mode:
-trial:
-  command:
-  codeDir:
-  gpuNum:
-#machineList can be empty if the platform is local
-machineList:
-  - ip:
-    port:
-    username:
-    passwd: 

examples/trials/mnist-pytorch/config_assessor.yml

-authorName: default
-experimentName: example_mnist_pytorch
-trialConcurrency: 1
-maxExecDuration: 1h
-maxTrialNum: 50
-#choice: local, remote
-trainingServicePlatform: local
-searchSpacePath: search_space.json
-#choice: true, false
-useAnnotation: false
+searchSpaceFile: search_space.json
+trialCommand: python3 mnist.py  # NOTE: change "python3" to "python" if you are using Windows
+trialGpuNumber: 0
+trialConcurrency: 4
+maxTrialNumber: 20
 tuner:
-  #choice: TPE, Random, Anneal, Evolution, BatchTuner, MetisTuner, GPTuner
-  #SMAC (SMAC should be installed through nnictl)
-  builtinTunerName: TPE
+  name: TPE
   classArgs:
-    #choice: maximize, minimize
     optimize_mode: maximize
 assessor:
-  #choice: Medianstop, Curvefitting
-  builtinAssessorName: Curvefitting
+  name: Curvefitting
   classArgs:
     epoch_num: 20
     threshold: 0.9
-trial:
-  command: python3 mnist.py
-  codeDir: .
-  gpuNum: 0
+trainingService:
+  platform: local