VLMo: Unified Vision-Language Pre-Training with Mixture-of-Modality-Experts - Bao, Hangbo; Wang, Wenhui; Dong, Li; Liu, Qiang; Mohammed, Owais Khan; Aggarwal, Kriti; Som, Subhojit; Piao, Songhao; Wei, Furu (Advances in Neural Information Processing Systems, 2022)