SPG: Sandwiched Policy Gradient for Masked Diffusion Language Models - Wang, Chenyu; Rashidinejad, Paria; Su, Andy (DiJia); Jiang, Song; Wang, Sid; Zhao, Siyan; Zhou, Cai; Shen, Shannon; Chen, Feiyu; Jaakkola, Tommi; Tian, Yuandong; Liu, Bo (International Conference on Learning Representations, 2026)