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Title: Low Bias Gradient Estimates for Very Deep Boolean Stochastic Networks

Paper content:
Stochastic neural networks with discrete random variables are an important class of models for their expressivity and interpretability.
Since direct differentiation and backpropagation is not possible, Monte Carlo gradient estimation techniques have been widely employed for training such models.
Efficient stochastic gradient estimators, such Straight-Through and Gumbel-Softmax, work well for shallow models with one or two stochastic layers.
Their performance, however, suffers with increasing model complexity.
In this work we focus on stochastic networks with multiple layers of Boolean latent variables.
To analyze such such networks, we employ the framework of harmonic analysis for Boolean functions.
We use it to derive an analytic formulation for the source of bias in the biased Straight-Through estimator.
Based on the analysis we propose \emph{FouST}, a simple gradient estimation algorithm that relies on three simple bias reduction steps.
Extensive experiments show that FouST performs favorably compared to state-of-the-art biased estimators, while being much faster than unbiased ones.
To the best of our knowledge FouST is the first gradient estimator to train up very deep stochastic neural networks, with up to 80 deterministic and 11 stochastic layers.
Stochastic neural networks with discrete latent variables have been an alluring class of models for their expressivity and interpretability, dating back to foundational work on Helmholtz machines (Dayan et al., 1995) and sigmoid belief nets (Neal, 1992) .
Since they are not directly differentiable, discrete random variables do not mesh well with the workhorse of modern Deep Learning, that is the backpropagation algorithm.
Monte Carlo gradient estimation is an effective solution where, instead of computing the true gradients, one can sample gradients from some distribution.
The sample estimates can be either biased or unbiased.
Unbiased gradient estimates like score function estimators (Williams, 1992) come typically at the cost of high variance leading to slow learning.
In contrast, biased gradient estimates such Straight-Through (Bengio et al., 2013) , while efficient, run the risk of convergence to poor minima and unstable training.
To this end several solutions have recently been proposed that either reduce variance in unbiased estimators (Mnih & Gregor, 2014; Gu et al., 2015; Tucker et al., 2017; Rezende et al., 2014; Grathwohl et al., 2017) or control bias in biased estimators (Jang et al., 2016; Maddison et al., 2016) .
These methods, however, have difficulty scaling up to complex neural networks with multiple stochastic layers: low-variance unbiased estimators are too expensive 1 , while the compounded bias from the continuous relaxations on multiple stochastic layers leads to poor minima.
In this work we focus on biased estimators.
Our goal in this paper is a gradient estimator for Boolean random variables that works for any complex -deep or wide-neural network architecture.
We resort to the term Boolean instead of binary to emphasize that we work directly on the Boolean space {−1, +1}, without any continuous relaxations or quantizations.
With this in mind we re-purpose the framework of harmonic analysis of Boolean functions, widely used in computational learning and computational complexity theory (O'Donnell, 2014; Linial et al., 1993; Mossel et al., 2003; Mansour, 1994) .
We cast stochastic neural networks as Boolean functions f (z) over Boolean latent variables z sampled from probability 1.
We introduce the framework of harmonic analysis of Boolean functions to analyze discrete stochastic neural networks and their REINFORCE and Straight-Through gradients.
We show that stochastic gradients compute Fourier coefficients.
2. Based on the above harmonic analysis we present FouST -a low-bias gradient estimator for Boolean latent variables based on three bias reduction steps.
As a side contribution, we show that the gradient estimator employed with DARN (Gregor et al., 2013) , originally proposed for autoregressive models, is a strong baseline for gradient estimation in large and complex models with many stochastic layers.
3. We show that FouST is amenable to complex stochastic neural networks with Boolean random variables.
To the best of our knowledge, FouST is the first gradient estimate algorithm that can train very deep stochastic neural networks with Boolean latent variables.
The practical outcome is a simple gradient estimate algorithm that can be plugged in complex stochastic neural networks with multiple layers of Boolean random variables.

assistant: text: We present a low-bias estimator for Boolean stochastic variable models with many stochastic layers.