Ensemble Neural Networks (ENN): A gradient-free stochastic method
This method addresses optimization challenges in neural networks for real-world engineering applications, particularly with small datasets, but appears incremental as it builds on existing ensemble and Bayesian techniques.
The authors tackled the problem of gradient-based optimization in neural networks by developing Ensemble Neural Networks (ENN), a gradient-free stochastic method that uses covariance matrices for optimization and performs uncertainty quantification, showing it performs much better than traditional Bayesian neural networks in experiments.
In this study, an efficient stochastic gradient-free method, the ensemble neural networks (ENN), is developed. In the ENN, the optimization process relies on covariance matrices rather than derivatives. The covariance matrices are calculated by the ensemble randomized maximum likelihood algorithm (EnRML), which is an inverse modeling method. The ENN is able to simultaneously provide estimations and perform uncertainty quantification since it is built under the Bayesian framework. The ENN is also robust to small training data size because the ensemble of stochastic realizations essentially enlarges the training dataset. This constitutes a desirable characteristic, especially for real-world engineering applications. In addition, the ENN does not require the calculation of gradients, which enables the use of complicated neuron models and loss functions in neural networks. We experimentally demonstrate benefits of the proposed model, in particular showing that the ENN performs much better than the traditional Bayesian neural networks (BNN). The EnRML in ENN is a substitution of gradient-based optimization algorithms, which means that it can be directly combined with the feed-forward process in other existing (deep) neural networks, such as convolutional neural networks (CNN) and recurrent neural networks (RNN), broadening future applications of the ENN.