DPOK: Reinforcement Learning for Fine-tuning Text-to-Image Diffusion Models - Fan, Ying; Watkins, Olivia; Du, Yuqing; Liu, Hao; Ryu, Moonkyung; Boutilier, Craig; Abbeel, Pieter; Ghavamzadeh, Mohammad; Lee, Kangwook; Lee, Kimin (Advances in Neural Information Processing Systems, 2023)