class_docstr: Plugin to show the scores of the true class during the lasts training
      epochs of each experience, averaged  by new and old classes.
code: def after_training_epoch(
      self, strategy: "SupervisedTemplate"
  ) -> "MetricResult":
      if strategy.clock.train_exp_epochs == strategy.train_epochs - 1:
          return self._package_result(strategy)
      else:
          return None
  
imports: from abc import ABC, abstractmethod
  from collections import defaultdict
  from typing import Callable, Dict, Set, TYPE_CHECKING, List, Optional, TypeVar
  import torch
  from matplotlib.axes import Axes
  from matplotlib.figure import Figure
  from matplotlib.pyplot import subplots
  from torch import Tensor, arange
  from avalanche.evaluation import Metric, PluginMetric
  from avalanche.evaluation.metric_utils import get_metric_name
  from avalanche.evaluation.metrics import Mean
  from avalanche.evaluation.metric_results import MetricValue, AlternativeValues
  from typing_extensions import Literal
label_desc: Use image node_id 3 for calling the MeanScoresTrainPluginMetric obj's underlying member method code with example usage: obj.after_training_epoch(strategy) and returns: self, None