Add guidance free method as GFT

Source code: https://github.com/thu-ml/GFT
Paper: Chen, Huayu, Kai Jiang, Kaiwen Zheng, Jianfei Chen, Hang Su, and Jun Zhu. “Visual Generation Without Guidance,” 2025. ICML, https://openreview.net/forum?id=gM6aboVgTO.
