In the paper 'Pretraining Methods for Dialog Context Representation Learning', it mentions: 1) generative transformers were the best models on both human and automated evaluations in
the 2nd ConvAI challenge; 2) NUR and NUG are complementary tasks. Over all of the results, we can see that pretraining with either NUG or NUR, gives strong results when fine-tuning on the other one. These two claims are both from another related paper that you've read. Provide the full name of that paper.