code: def test_main(self, mock_grid):
      mock_grid.return_value = 10
  
      callback = imaging.MakeGrid(
          key="x",
          num_images=18,
          nrow=9,
          padding=3,
          normalize=True,
          norm_range="tmp",
          scale_each=True,
          pad_value=1,
      )
  
      res = callback.on_cache("test", {})
      mock_grid.assert_called_once_with(
          "test",
          nrow=9,
          padding=3,
          normalize=True,
          range="tmp",
          scale_each=True,
          pad_value=1,
      )
      self.assertTrue(res == 10)
  
imports: from unittest import TestCase
  from mock import MagicMock, patch, ANY
  import torch
  import torchbearer
  import torchbearer.callbacks.imaging
  import matplotlib.pyplot
label_desc: Use image node_id 1 for calling the TestMakeGrid obj's underlying member method code with example usage: obj.test_main(mock_grid) without return types