DIVA-GRPO: Enhancing Multimodal Reasoning through Difficulty-Adaptive Variant Advantage - Gao, Haowen; zhang, zhenyu; Pang, Liang; Guo, Fangda; dou, hongjian; Lv, Guannan; Liu, ShaoGuo; Gao, Tingting; Shen, Huawei; Cheng, Xueqi (International Conference on Learning Representations, 2026)