I. Introduction
In the past few years, we have witnessed the rapid development of machine learning (ML) in the field of artificial intelligence (AI) applications, such as computer vision, automatic speech recognition, natural language processing and recommendation system [1]–[3]. The success of these machine learning technologies, especially deep learning (DL), builds on a large volume of data (i.e., big data). With the advent of the Internet of Things (IoT), massive data is collected by Internet connected smart devices with limited resources (e.g., smartphones, sensors, etc.). In most traditional ML technologies, the local data collected by smart devices need to be transmitted and processed at a cloud or data center to train effective inference models. However, this causes excessive computation and storage costs, and the smart devices also suffer from serious privacy leakage risk [4].