I. Introduction
Ever since Hinton [1] published his phenomenal article in 2007, renewed interest in artificial neural networks (ANNs) has contributed significantly to the progress of artificial intelligence (AI) and led to the explosive growth of deep learning. As one of the hottest research topics today, AI has generated widespread interest from both industry and academia, simultaneously causing remarkable social impact (e.g., Google’s AlphaGo [2]). Various tasks ranging from visual recognition, autonomous driving, voice assistance, chess playing, and many others have already been realized, and some AI applications have even achieved better cognitive accuracy than humans [3].