code: def test_forgetting(self):
      metric = Forgetting()
      f = metric.result()
      self.assertEqual(f, {})
      f = metric.result_key(k=0)
      self.assertIsNone(f)
      metric.update(0, 1, initial=True)
      f = metric.result_key(k=0)
      self.assertIsNone(f)
      metric.update(0, 0.4)
      f = metric.result_key(k=0)
      self.assertEqual(f, 0.6)
      metric.reset()
      self.assertEqual(metric.result(), {})
  
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 24 for calling the GeneralMetricTests obj's underlying member method code with example usage: obj.test_forgetting() without return types