not quite what you are looking for, but this is an interesting paper on unsupervised summ.
which uses gpt-2 as a kind of pseudo summrizer, then score the summaries generated by it using two other models for fluency and coverage and train the summarizer using reinforcement learning to maximize the fluency and coverage score 

  
      aclweb.org
  
  
    
2020.acl-main.460.pdf 3

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