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Title: Representing Model Uncertainty of Neural Networks in Sparse Information Form

Paper content:
This paper addresses the problem of representing a system's belief using multi-variate normal distributions (MND) where the underlying model is based on a deep neural network (DNN).
The major challenge with DNNs is the computational complexity that is needed to obtain model uncertainty using MNDs.
To achieve a scalable method, we propose a novel approach that expresses the parameter posterior in sparse information form.
Our inference algorithm is based on a novel Laplace Approximation scheme, which involves a diagonal correction of the Kronecker-factored eigenbasis.
As this makes the inversion of the information matrix intractable - an operation that is required for full Bayesian analysis, we devise a low-rank   approximation of this eigenbasis and a memory-efficient sampling scheme.
We provide both a theoretical analysis and an empirical evaluation on various benchmark data sets, showing the superiority of our approach over existing methods.
Whenever machine learning methods are used for safety-critical applications such as medical image analysis or autonomous driving, it is crucial to provide a precise estimation of the failure probability of the learned predictor.
Therefore, most of the current learning approaches return distributions rather than single, most-likely predictions.
For example, DNNs trained for classification usually use the softmax function to provide a distribution over predicted class labels.
Unfortunately, this method tends to severely underestimate the true failure probability, leading to overconfident predictions (Guo et al., 2017) .
The main reason for this is that neural networks are typically trained with a principle of maximum likelihood, neglecting their epistemic or model uncertainty with the point estimates.
A widely known work by Gal (2016) shows that this can be mitigated by using dropout at test time.
This so-called Monte-Carlo dropout (MC-dropout) has the advantage that it is relatively easy to use and therefore very popular in practice.
However, MC-dropout also has significant drawbacks.
First, it requires a specific stochastic regularization during training.
This limits its use on already well trained architectures, because current networks are often trained with other regularization techniques such as batch normalization.
Moreover, it uses a Bernoulli distribution to represent the complex model uncertainty, which in return, leads to an underestimation of the predictive uncertainty.
Several strong alternatives exist without these drawbacks.
Variational inference Kingma et al., 2015; Graves, 2011) and expectation propagation (Herandez-Lobato & Adams, 2015) are such examples.
Yet, these methods use a diagonal covariance matrix which limits their applicability as the model parameters are often highly correlated.
Building upon these, Sun et al. (2017) ; Louizos & Welling (2016) ; Zhang et al. (2018) ; Ritter et al. (2018a) show that the correlations between the parameters can also be computed efficiently by decomposing the covariance matrix of MND into Kronecker products of smaller matrices.
However, not all matrices can be Kronecker decomposed and thus, these simplifications usually induce crude approximations (Bae et al., 2018) .
As the dimensionality of statistical manifolds are prohibitively too large in DNNs, more expressive, efficient but still easy to use ways of representing such high dimensional distributions are required.
To tackle this challenge, we propose to represent the model uncertainty in sparse information form of MND.
As a first step, we devise a new Laplace Approximation (LA) for DNNs, in which we improve the state-of-the-art Kronecker factored approximations of the Hessian (George et al., 2018) by correcting the diagonal variance in parameter space.
We show that these can be computed efficiently, and that the information matrix of the resulting parameter posterior is more accurate in terms of the Frobenius norm.
In this way the model uncertainty is approximated in information form of the MND.
counts [-] Figure 1: Main idea.
(a) Covariance matrix Σ for DNNs is intractable to infer, store and sample (an example taken from our MNIST experiments).
(b) Our main insight is that the spectrum (eigenvalues) of information matrix (inverse of covariance) tend to be sparse.
(c) Exploiting this insight a Laplace Approximation scheme is devised which applies a spectral sparsification (LRA) while keeping the diagonals exact.
With this formulation, the complexity becomes tractable for sampling while producing more accurate estimates.
Here, the diagonal elements (nodes in graphical interpretation) corresponds to information content in a parameter whereas the corrections (links) are the off-diagonals.
As this results in intractable inverse operation for sampling, we further propose a novel low-rank representation of the resulting Kronecker factorization, which paves the way to applications on large network structures trained on realistically sized data sets.
To realize such sparsification, we propose a novel algorithm that enables a low-rank approximation of the Kronecker factored eigenvalue decomposition, and we demonstrate an associated sampling computations.
Our experiments demonstrate that our approach is effective in providing more accurate uncertainty estimates and calibration on considered benchmark data sets.
A detailed theoretical analysis is also provided for further insights.
We summarize our main contributions below.
• A novel Laplace Approximation scheme with a diagonal correction to the eigenvalue rescaled approximations of the Hessian, as a practical inference tool (section 2.2).
• A novel low-rank representation of Kronecker factored eigendecomposition that preserves Kronecker structure (section 2.3).
This results in a sparse information form of MND.
• A novel algorithm to enable a low rank approximation (LRA) for the given representation of MND (algorithm 1) and derivation of a memory-wise tractable sampler (section B.2).
• Both theoretical (section C) and experimental results (section 4) showing the applicability of our approach.
In our experiments, we showcase the state-of-the-art performance within the class of Bayesian Neural Networks that are scalable and training-free.
To our knowledge we explore a sparse information form to represent the model uncertainty of DNNs for the first time.
Figure 1 depicts our main idea which we provide more rigorous formulation next.
We address an effective approach of representing model uncertainty in deep neural networks using Multivariate Normal Distribution, which has been thought computationally intractable so far.
This is achieved by designing its novel sparse information form.
With one of the most expressive representation of model uncertainty in current Bayesian deep learning literature, we show that uncertainty can be estimated more accurately than existing methods.
For future works, we plan to demonstrate a real world application of this approach, pushing beyond the validity of concepts.

assistant: text: An approximate inference algorithm for deep learning