code: def test_mean(self):
      metric = Mean()
      self.assertEqual(metric.result(), 0)
      metric.update(0.1, 1)
      self.assertEqual(metric.result(), 0.1)
      metric.reset()
      self.assertEqual(metric.result(), 0)
  
imports: import unittest
  import torch
  from avalanche.evaluation.metrics import TaskAwareAccuracy, TopkAccuracy, AverageMeanClassAccuracy, MultiStreamAMCA, ClassAccuracy, TaskAwareLoss, ConfusionMatrix, DiskUsage, MAC, CPUUsage, MaxGPU, MaxRAM, Mean, Sum, ElapsedTime, Forgetting, ForwardTransfer, CumulativeAccuracy, LabelsRepartition
  from tests.unit_tests_utils import FAST_TEST, is_github_action
label_desc: Use image node_id 22 for calling the GeneralMetricTests obj's underlying member method code with example usage: obj.test_mean() without return types