text: Title: Add convenience methods to export and import fine-tuned prompts
  Body: I can run["deep" prompt-tuning](https://github.com/bigscience-workshop/petals/blob/main/examples/prompt-tuning-personachat.ipynb) successfully.
  
  but, how can I export the prefix tokens from the Petals client? 
  and, how can I use the prefix tokens which export from Petals client when I want to do some inference job? 
  
  I appreciate you help