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 before_training_epoch(
      self, strategy: "SupervisedTemplate"
  ) -> None:
      self.reset()
      self.update_new_classes(strategy)
  
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 1 for calling the MeanScoresTrainPluginMetric obj's underlying member method code with example usage: obj.before_training_epoch(strategy) without return types