code: def on_test_start(self):
      self.reporter._testing = True
  
imports: import unittest
  from typing import List
  import pytorch_lightning
  import torch
  import torch.nn
  import torch.optim
  from reagent.reporting import CompoundReporter, ReporterBase
  from reagent.training import MultiStageTrainer, ReAgentLightningModule
  from torch.utils.data import DataLoader, TensorDataset
label_desc: Use image node_id 3 for calling the DummyTrainer obj's underlying member method code with example usage: obj.on_test_start() without return types