class_docstr: Flattens the entire conversation history.
  
  Simply concatenates all turns in the conversation with a newline. Frequently useful
  when composed with other mutators.
code: def many_episode_mutation(
      self, episode: List[Message]
  ) -> List[List[Message]]:
      history = []
      for message in episode:
          history.append(message.pop("text"))
          message["text"] = "\n".join(history)
          yield [message]
          history.append(random.choice(message["labels"]))
  
imports: import random
  from typing import List
  from parlai.core.message import Message
  from parlai.core.mutators import ManyEpisodeMutator, register_mutator
label_desc: Use image node_id 1 for calling the FlattenMutator obj's underlying member method code with example usage: obj.many_episode_mutation(episode) without return types