Arbel, Michael; (2021) Methods for Optimization and Regularization of Generative Models. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis studies the problem of regularizing and optimizing generative models, often using insights and techniques from kernel methods. The work proceeds in three main themes. Conditional score estimation. We propose a method for estimating conditional densities based on a rich class of RKHS exponential family models. The algorithm works by solving a convex quadratic problem for fitting the gradient of the log density, the score, thus avoiding the need for estimating the normalizing constant. We show the resulting estimator to be consistent and provide convergence rates when the model is well-specified. Structuring and regularizing implicit generative models. In a first contribution, we introduce a method for learning Generative Adversarial Networks, a class of Implicit Generative Models, using a parametric family of Maximum Mean Discrepancies (MMD). We show that controlling the gradient of the critic function defining the MMD is vital for having a sensible loss function. Moreover, we devise a method to enforce exact, analytical gradient constraints. As a second contribution, we introduce and study a new generative model suited for data with low intrinsic dimension embedded in a high dimensional space. This model combines two components: an implicit model, which can learn the low-dimensional support of data, and an energy function, to refine the probability mass by importance sampling on the support of the implicit model. We further introduce algorithms for learning such a hybrid model and for efficient sampling. Optimizing implicit generative models. We first study the Wasserstein gradient flow of the Maximum Mean Discrepancy in a non-parametric setting and provide smoothness conditions on the trajectory of the flow to ensure global convergence. We identify cases when this condition does not hold and propose a new algorithm based on noise injection to mitigate this problem. In a second contribution, we consider the Wasserstein gradient flow of generic loss functionals in a parametric setting. This flow is invariant to the model's parameterization, just like the Fisher gradient flows in information geometry. It has the additional benefit to be well defined even for models with varying supports, which is particularly well suited for implicit generative models. We then introduce a general framework for approximating the Wasserstein natural gradient by leveraging a dual formulation of the Wasserstein pseudo-Riemannian metric that we restrict to a Reproducing Kernel Hilbert Space. The resulting estimator is scalable and provably consistent as it relies on Nystrom methods.

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