code: def testClassErrorMeter(self):
      mtr = meter.ClassErrorMeter(topk=[1])
      output = torch.eye(3)
      target = torch.range(0, 2)
      mtr.add(output, target)
      err = mtr.value()
  
      self.assertEqual(err, [0], "All should be correct")
  
      target[0] = 1
      target[1] = 0
      target[2] = 0
      mtr.add(output, target)
      err = mtr.value()
      self.assertEqual(err, [50.0], "Half should be correct")
  
file_path: pywick/tests/test_meters.py
imports: import unittest
  import math
  import torch
  import pywick.meters
  import numpy
label_desc: Use image node_id 2 for calling the TestMeters obj's underlying member method code with example usage: obj.testClassErrorMeter() without return types