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Title: Sampling-Free Learning of Bayesian Quantized Neural Networks

Paper content:
Bayesian learning of model parameters in neural networks is important in scenarios where estimates with well-calibrated uncertainty are important.
In this paper, we propose Bayesian quantized networks (BQNs), quantized neural networks (QNNs) for which we learn a posterior distribution over their discrete parameters.
We provide a set of efficient algorithms for learning and prediction in BQNs without the need to sample from their parameters or activations, which not only allows for differentiable learning in quantized models but also reduces the variance in gradients estimation.
We evaluate BQNs on MNIST, Fashion-MNIST and KMNIST classification datasets compared against bootstrap ensemble of QNNs (E-QNN).
We demonstrate BQNs achieve both lower predictive errors and better-calibrated uncertainties than E-QNN (with less than 20% of the negative log-likelihood).
A Bayesian approach to deep learning considers the network's parameters to be random variables and seeks to infer their posterior distribution given the training data.
Models trained this way, called Bayesian neural networks (BNNs) (Wang & Yeung, 2016) , in principle have well-calibrated uncertainties when they make predictions, which is important in scenarios such as active learning and reinforcement learning (Gal, 2016) .
Furthermore, the posterior distribution over the model parameters provides valuable information for evaluation and compression of neural networks.
There are three main challenges in using BNNs: (1) Intractable posterior: Computing and storing the exact posterior distribution over the network weights is intractable due to the complexity and high-dimensionality of deep networks.
(2) Prediction: Performing a forward pass (a.k.a. as probabilistic propagation) in a BNN to compute a prediction for an input cannot be performed exactly, since the distribution of hidden activations at each layer is intractable to compute.
(3) Learning:
The classic evidence lower bound (ELBO) learning objective for training BNNs is not amenable to backpropagation as the ELBO is not an explicit function of the output of probabilistic propagation.
These challenges are typically addressed either by making simplifying assumptions about the distributions of the parameters and activations, or by using sampling-based approaches, which are expensive and unreliable (likely to overestimate the uncertainties in predictions).
Our goal is to propose a sampling-free method which uses probabilistic propagation to deterministically learn BNNs.
A seemingly unrelated area of deep learning research is that of quantized neural networks (QNNs), which offer advantages of computational and memory efficiency compared to continuous-valued models.
QNNs, like BNNs, face challenges in training, though for different reasons: (4.1) The non-differentiable activation function is not amenable to backpropagation.
(4.2)
Gradient updates cease to be meaningful, since the model parameters in QNNs are coarsely quantized.
In this work, we combine the ideas of BNNs and QNNs in a novel way that addresses the aforementioned challenges (1)(2)(3)(4) in training both models.
We propose Bayesian quantized networks (BQNs), models that (like QNNs) have quantized parameters and activations over which they learn (like BNNs) categorical posterior distributions.
BQNs have several appealing properties:
• BQNs solve challenge (1) due to their use of categorical distributions for their model parameters.
• BQNs can be trained via sampling-free backpropagation and stochastic gradient ascent of a differentiable lower bound to ELBO, which addresses challenges (2), (3) and (4) above.
• BQNs leverage efficient tensor operations for probabilistic propagation, further addressing challenge (2).
We show the equivalence between probabilistic propagation in BQNs and tensor contractions (Kolda & Bader, 2009) , and introduce a rank-1 CP tensor decomposition (mean-field approximation) that speeds up the forward pass in BQNs.
• BQNs provide a tunable trade-off between computational resource and model complexity: using a refined quantization allows for more complex distribution at the cost of more computation.
• Sampling from a learned BQN provides an alternative way to obtain deterministic QNNs .
In our experiments, we demonstrate the expressive power of BQNs.
We show that BQNs trained using our sampling-free method have much better-calibrated uncertainty compared with the stateof-the-art Bootstrap ensemble of quantized neural networks (E-QNN) trained by Courbariaux et al. (2016) .
More impressively, our trained BQNs achieve comparable log-likelihood against Gaussian Bayesian neural network (BNN) trained with stochastic gradient variational Bayes (SGVB) (Shridhar et al., 2019) (the performance of Gaussian BNNs are expected to be better than BQNs since they allows for continuous random variables).
We further verify that BQNs can be easily used to compress (Bayesian) neural networks and obtain determinstic QNNs.
Finally, we evaluate the effect of mean-field approximation in BQN, by comparing with its Monte-Carlo realizations, where no approximation is used.
We show that our sampling-free probabilistic propagation achieves similar accuracy and log-likelihood -justifying the use of mean-field approximation in BQNs.
We present a sampling-free, backpropagation-compatible, variational-inference-based approach for learning Bayesian quantized neural networks (BQNs).
We develop a suite of algorithms for efficient inference in BQNs such that our approach scales to large problems.
We evaluate our BQNs by Monte-Carlo sampling, which proves that our approach is able to learn a proper posterior distribution on QNNs.
Furthermore, we show that our approach can also be used to learn (ensemble) QNNs by taking maximum a posterior (or sampling from) the posterior distribution.
assuming g n (φ) can be (approximately) computed by sampling-free probabilistic propagation as in Section 2.
However, this approach has two major limitations:
(a) the Bayes' rule needed to be derived case by case, and analytic rule for most common cases are not known yet.
(b) it is not compatible to modern optimization methods (such as SGD or ADAM) as the optimization is solved analytically for each data point, therefore difficult to cope with large-scale models.
(2) Sampling-based Variational inference (SVI), formulates an optimization problem and solves it approximately via stochastic gradient descent (SGD).
The most popular method among all is, Stochastic Gradient Variational Bayes (SGVB), which approximates L n (φ) by the average of multiple samples (Graves, 2011; Blundell et al., 2015; Shridhar et al., 2019) .
Before each step of learning or prediction, a number of independent samples of the model parameters {θ s } S s=1 are drawn according to the current estimate of Q, i.e. θ s ∼ Q, by which the predictive function g n (φ) and the loss L n (φ) can be approximated by
where f n (θ) = Pr[y n |x n , θ] denotes the predictive function given a specific realization θ of the model parameters.
The gradients of L n (φ) can now be approximated as
This approach has multiple drawbacks:
(a) Repeated sampling suffers from high variance, besides being computationally expensive in both learning and prediction phases;
(b) While g n (φ) is differentiable w.r.t. φ, f n (θ) may not be differentiable w.r.t. θ.
One such example is quantized neural networks, whose backpropagation is approximated by straight through estimator (Bengio et al., 2013
Our approach considers a wider scope of problem settings, where the model could be stochastic, i.e.
] is an arbitrary function.
Furthermore, Wu et al. (2018) considers the case that all parameters θ are Gaussian distributed, whose sampling-free probabilistic propagation requires complicated approximation (Shekhovtsov & Flach, 2018) .
Quantized Neural Networks These models can be categorized into two classes: (1) Partially quantized networks, where only weights are discretized (Han et al., 2015; Zhu et al., 2016) ; (2) Fully quantized networks, where both weights and hidden units are quantized (Courbariaux et al., 2015; Kim & Smaragdis, 2016; Zhou et al., 2016; Rastegari et al., 2016; Hubara et al., 2017) .
While both classes provide compact size, low-precision neural network models, fully quantized networks further enjoy fast computation provided by specialized bit-wise operations.
In general, quantized neural networks are difficult to train due to their non-differentiability.
Gradient descent by backpropagation is approximated by either straight-through estimators (Bengio et al., 2013) or probabilistic methods (Esser et al., 2015; Shayer et al., 2017; Peters & Welling, 2018) .
Unlike these papers, we focus on Bayesian learning of fully quantized networks in this paper.
Optimization of quantized neural networks typically requires dedicated loss function, learning scheduling and initialization.
For example, Peters & Welling (2018) considers pre-training of a continuous-valued neural network as the initialization.
Since our approach considers learning from scratch (with an uniform initialization), the performance could be inferior to prior works in terms of absolute accuracy.
Tensor Networks and Tensorial Neural Networks Tensor networks (TNs) are widely used in numerical analysis (Grasedyck et al., 2013) , quantum physiscs (Orús, 2014), and recently machine learning (Cichocki et al., 2016; 2017) to model interactions among multi-dimensional random objects.
Various tensorial neural networks (TNNs) (Su et al., 2018; Newman et al., 2018) have been proposed that reduce the size of neural networks by replacing the linear layers with TNs.
Recently, (Robeva & Seigal, 2017) points out the duality between probabilistic graphical models (PGMs) and TNs.
I.e. there exists a bijection between PGMs and TNs.
Our paper advances this line of thinking by connecting hierarchical Bayesian models (e.g. Bayesian neural networks) and hierarchical TNs.

assistant: text: We propose Bayesian quantized networks, for which we learn a posterior distribution over their quantized parameters.