class_docstr: Average the scores of the true class by old and new classes
code: def update_new_classes(self, new_classes: Set[int]):
      self.new_classes.update(new_classes)
  
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 MeanNewOldScores obj's underlying member method code with example usage: obj.update_new_classes(new_classes) without return types