code: def episode_mutation(self, episode: List[Message]) -> List[Message]:
      last_knowledge = None
      for msg in episode:
          knowledge = msg.pop("available_knowledge_text")
          if last_knowledge != knowledge:
              msg["available_knowledge_text"] = knowledge
          last_knowledge = knowledge
      return episode
  
imports: import copy
  import json
  import os
  from enum import Enum
  from typing import List, Optional, Tuple
  from parlai.core.message import Message
  from parlai.core.metrics import F1Metric
  from parlai.core.mutators import EpisodeMutator, MessageMutator, register_mutator
  from parlai.core.opt import Opt
  from parlai.core.params import ParlaiParser
  from parlai.core.teachers import DialogTeacher
  from parlai.tasks.cmu_dog.build import build
  from parlai.tasks.wizard_of_wikipedia.agents import RareWordF1Calculator, TOKEN_KNOWLEDGE, TOKEN_END_KNOWLEDGE
  from parlai.utils.data import DatatypeHelper
  from parlai.utils.io import PathManager
  from parlai.utils.logging import logger
  from parlai.utils.typing import TShared
label_desc: Use image node_id 1 for calling the KnowledgeWhenUpdatedMutator obj's underlying member method code with example usage: obj.episode_mutation(episode) and returns: episode