code: def test_depth_none(self):
      mm = ManifoldMixup().at_depth(None)
      state = {torchbearer.MODEL: self.model}
      mm.on_start(state)
  
      self.assertTrue(len(mm._layers) == 12)
  
imports: from unittest import TestCase
  from mock import patch, Mock
  from torch import nn
  from torchbearer.callbacks.manifold_mixup import ManifoldMixup
  import torchbearer
  import torch
label_desc: Use image node_id 2 for calling the TestManifoldMixup obj's underlying member method code with example usage: obj.test_depth_none() without return types