code: def test_best_checkpointer(self):
      model = _SimpleModel()
      dataloader = self._data_loader("cpu")
      opt = torch.optim.SGD(model.parameters(), 0.1)
      metric_name = "metric"
      total_iter = 40
      test_period = 10
      test_cases = [
          ("max", iter([0.3, 0.4, 0.35, 0.5]), 3),
          ("min", iter([1.0, 0.8, 0.9, 0.9]), 2),
          ("min", iter([math.nan, 0.8, 0.9, 0.9]), 1),
      ]
      for mode, metrics, call_count in test_cases:
          trainer = SimpleTrainer(model, dataloader, opt)
          with tempfile.TemporaryDirectory(
              prefix="detectron2_test"
          ) as d:
              checkpointer = Checkpointer(
                  model, d, opt=opt, trainer=trainer
              )
              trainer.register_hooks(
                  [
                      hooks.EvalHook(
                          test_period,
                          lambda: {metric_name: next(metrics)},
                      ),
                      hooks.BestCheckpointer(
                          test_period,
                          checkpointer,
                          metric_name,
                          mode=mode,
                      ),
                  ]
              )
              with mock.patch.object(
                  checkpointer, "save"
              ) as mock_save_method:
                  trainer.train(0, total_iter)
                  self.assertEqual(
                      mock_save_method.call_count, call_count
                  )
  
imports: import json
  import math
  import os
  import tempfile
  import time
  import unittest
  from unittest import mock
  import torch
  from fvcore.common.checkpoint import Checkpointer
  from torch import nn
  from detectron2 import model_zoo
  from detectron2.config import configurable, get_cfg
  from detectron2.engine import DefaultTrainer, SimpleTrainer, default_setup, hooks
  from detectron2.modeling.meta_arch import META_ARCH_REGISTRY
  from detectron2.utils.events import CommonMetricPrinter, JSONWriter
label_desc: Use image node_id 11 for calling the TestTrainer obj's underlying member method code with example usage: obj.test_best_checkpointer() without return types