I. Introduction
In the past decade, with the explosive growth of data, the traditional machine learning (ML) methods cannot make full use of big data and fail to provide ML models with superior performance. Deep learning, as a new research direction in the field of ML, has significantly improved performance over traditional ML. Therefore, it has been widely used in image classification [1], [2], semantic segmentation [3], [4], object detection [5], [6], and so on. Image classification is one of the most popular applications of deep learning. Some security-critical applications highly rely on image classification techniques. For example, autonomous driving vehicles employ images captured by cameras to classify road traffic signs (such as Stop signs and Speed Limit signs).