Zihang Tian Rui Li Jingsen Zhang Xiaohe Bo Wei Huo Xu Chen HAPS: Hierarchical LLM Routing with Joint Architecture and Parameter Search. 2026 January abs/2601.05903 CoRR https://doi.org/10.48550/arXiv.2601.05903 db/journals/corr/corr2601.html#abs-2601-05903 streams/journals/corr