Hierarchical Representations for Efficient Architecture Search
Deep Neural Networks (DNN) have shown remarkable performance in several areas such as computer vision, natural language processing, among others; however, improvements over previous benchmarks have required extensive research and experimentation by domain experts. In DNN the composition of linear and nonlinear functions produces internal representations of data who are in most cases better than handcrafted ones; consequently, researchers using Deep Learning techniques have lately shifted their focus from working on input features to designing optimal DNN architectures. However, the quest for finding an optimal DNN architecture by combining layers and modules requires frequent trial and error experiments, a task that resembles the previous work on looking for handcrafted optimal features. As researchers aim to solve more difficult challenges the complexity of the resulting DNN is also increasing; therefore, some studies are introducing the use of automated techniques focused on searching for optimal architectures.
Lately, the use of algorithms for finding optimal DNN architectures has attracted the attention of researchers who have tackled the problem through four main groups of techniques. The first operates over the random DNN candidate’s weights and involves the use of an auxiliary HyperNetwork which maps architectures to feasible sets of weights and consequently allows an early evaluation of random DNN candidates. The second technique is Monte Carlo Tree Search (MCTS) who repeatedly narrows the search space by focusing on the most promising architectures previously seen. The third group of techniques use evolutionary algorithms where a fitness criteria is applied to filter the initial population of DNN candidates, then new individuals are added to the population by selecting the best performing ones and modifying them with one or several random mutations [Real, 2017]. The fourth and last group of techniques implement Reinforcement Learning where a policy based controller seeks to optimize the expected accuracy of new architectures based on rewards (accuracy) gained from previous proposals in the architecture space. From these four groups of techniques, Reinforcement Learning has offered the best experimental results; however, the paper we are summarizing implements evolutionary algorithms as its main approach.
Despite the technique used to look for an optimal architecture, searching in the architecture space usually requires the training and evaluation of many DNN candidates; therefore, it demands huge computational resources and pose a significant limitation for practical applications. Consequently, most techniques narrow the search space with predefined heuristics, either at the beginning or dynamically during the searching process. In the paper we are summarizing, the authors reduce the number of feasible architectures by forcing a hierarchical structure between network components. In other words, each DNN suggested as a candidate is formed by combining basic building blocks to form small modules, then the same basic structures introduced on the building blocks are used to combine and stack networks on the upper levels of the hierarchy. This approach allows the searching algorithm to sample highly complex and modularized networks similar to Inception or ResNet.
Despite some weaknesses regarding the efficiency of evolutionary algorithms, this study reveals that in fact these techniques can generate architectures who show competitive performance when a narrowing strategy is imposed over the search space. Accordingly, the main contributions of this paper are a well defined set of hierarchical representations who acts as the filtering criteria to pick DNN candidates and a novel evolutionary algorithm who produces image classifiers that achieve state of the art performance among similar evolutionary-based techniques.
Flat architecture representation
All the evaluated network architectures are directed acyclic graphs with only one source and one sink. Each node in the network represents a feature map and consequently each directed edge represents an operation that takes the feature map in the departing node as input and outputs a feature map on the arriving node. Under the previous assumption, any given architecture in the narrowed search space is formally expressed as a graph assembled by a series of operations (edges) among a defined set of adjacent feature maps (nodes).