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Title: Gaussian Process Meta-Representations For Hierarchical Neural Network Weight Priors

Paper content:
Bayesian inference offers a theoretically grounded and general way to train neural networks and can potentially give calibrated uncertainty.
However, it is challenging to specify a meaningful and tractable prior over the network parameters, and deal with the weight correlations in the posterior.
To this end, this paper introduces two innovations:
(i) a Gaussian process-based hierarchical model for the network parameters based on recently introduced unit embeddings that can flexibly encode weight structures, and
(ii) input-dependent contextual variables for the weight prior that can provide convenient ways to regularize the function space being modeled by the network through the use of kernels.
We show these models provide desirable test-time uncertainty estimates, demonstrate cases of modeling inductive biases for neural networks with kernels and demonstrate competitive predictive performance on an active learning benchmark.
The question of which priors one should use for Bayesian neural networks is largely unanswered, as two considerations need to be balanced: First, we want to keep inference in the high dimensional weight posterior tractable; Second, we desire to express our beliefs about the properties of the modeled functions compactly by modeling the collection of weights.
Especially the latter is typically hard, as functional regularization for weight-based models is non-trivial.
In order to cope with richer posterior inference than mean-field typically achieves, a variety of structured posterior models have been proposed recently, for instance utilizing radial posteriors (Oh et al., 2019) , or rich weight posteriors based on Gaussian processes (Louizos and Welling, 2016) .
When it comes to modeling priors on weights with correlations, recent work has attempted to capture feature-level correlations using for instance a horseshoe prior (Ghosh et al., 2018) .
One interesting direction of inquiry has focused on utilizing hyper-networks in order to model distributions over weights for an entire network (Ha et al., 2016; Pradier et al., 2018) , or alternatively to utilize unit-level level variables combined with compact hyper-networks to regress to single weights and capture weight correlations through the auxiliary variables (Karaletsos et al., 2018) .
We propose to tackle some of the challenges in modeling weight priors by extending the latter work and combining it with ideas from the Gaussian process literature to replace the hyper-network with a Gaussian process prior over weights.
We explore the use of compositional kernels to add input-dependence to the prior for our model and obtain rich models with beneficial properties in tasks such as active learning, and generalization, while maintaining tractable inference properties.

assistant: text: We introduce a Gaussian Process Prior over weights in a neural network and explore its ability to model input-dependent weights with benefits to various tasks, including uncertainty estimation and generalization in the low-sample setting.