code: def test_step(self):
      result = self._mean._step(self._cache)
      self.assertEqual(self._target, result)
  
imports: import unittest
  from mock import Mock, call
  from torchbearer.metrics import RunningMean, Metric, RunningMetric, Mean, Std, Var
  import torch
label_desc: Use image node_id 3 for calling the TestRunningMean obj's underlying member method code with example usage: obj.test_step() without return types