code: def test_cache_one_step(self):
      self._metric.train()
      for i in range(6):
          self._metric.process({})
      self._metric._step.assert_has_calls(
          [call([3]), call([3, 3, 3, 3, 3])]
      )
  
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 TestRunningMetric obj's underlying member method code with example usage: obj.test_cache_one_step() without return types