Towards Improved Variational Inference for Deep Bayesian Models
Sebastian W. Ober
Deep learning has revolutionized the last decade, being at the forefront of
extraordinary advances in a wide range of tasks including computer vision,
natural language processing, and reinforcement learning, to name but a few.
However, it is well-known that deep models trained via maximum likelihood
estimation tend to be overconfident and give poorly-calibrated predictions.
Bayesian deep learning attempts to address this by placing priors on the model
parameters, which are then combined with a likelihood to perform posterior
inference. Unfortunately, for deep models, the true posterior is intractable,
forcing the user to resort to approximations. In this thesis, we explore the
use of variational inference (VI) as an approximation, as it is unique in
simultaneously approximating the posterior and providing a lower bound to the
marginal likelihood. If tight enough, this lower bound can be used to optimize
hyperparameters and to facilitate model selection. However, this capacity has
rarely been used to its full extent for Bayesian neural networks, likely
because the approximate posteriors typically used in practice can lack the
flexibility to effectively bound the marginal likelihood. We therefore explore
three aspects of Bayesian learning for deep models: 1) we ask whether it is
necessary to perform inference over as many parameters as possible, or whether
it is reasonable to treat many of them as optimizable hyperparameters; 2) we
propose a variational posterior that provides a unified view of inference in
Bayesian neural networks and deep Gaussian processes; 3) we demonstrate how VI
can be improved in certain deep Gaussian process models by analytically
removing symmetries from the posterior, and performing inference on Gram
matrices instead of features. We hope that our contributions will provide a
stepping stone to fully realize the promises of VI in the future.