High Probability Bounds for a Class of Nonconvex Algorithms with AdaGrad Stepsize
Authors: Ali Kavis,Kfir Yehuda Levy,Volkan Cevher
ICLR 2022,Poster

Abstract: In this paper, we propose a new, simplified high probability analysis of AdaGrad for smooth, non-convex problems. 
More specifically, we focus on a particular accelerated gradient (AGD) template (Lan, 2020), through which we recover the original AdaGrad and its variant with averaging, and prove a convergence rate of $\mathcal O (1/ \sqrt{T})$ with high probability without the knowledge of smoothness and variance. 
We use a particular version of Freedman's concentration bound for martingale difference sequences (Kakade & Tewari, 2008) which enables us to achieve the best-known dependence of $\log (1 / \delta )$ on the probability margin $\delta$. 
We present our analysis in a modular way and obtain a complementary $\mathcal O (1 / T)$ convergence rate in the deterministic setting. 
To the best of our knowledge, this is the first high probability result for AdaGrad with a truly adaptive scheme, i.e., completely oblivious to the knowledge of smoothness and uniform variance bound, which simultaneously has best-known dependence of $\log( 1/ \delta)$. 
We further prove noise adaptation property of AdaGrad under additional noise assumptions.