用深度序列模型的结构特性改进概率推理,让模型更懂不确定性。
Probabilistic Learning and Generation in Deep Sequence Models
- 利用Transformer与稀疏高斯过程的相似性设计贝叶斯推断方法
- 用HiPPO机制在在线学习中高效记忆历史信息
- 为生成模型的潜变量引入自监督信号,提升生成质量
尽管深度序列模型(DSMs)预测性能优异,但部署时普遍缺乏对不确定性的感知。相比之下,概率模型通过概率规则量化未观测变量的不确定性。贝叶斯方法基于贝叶斯定理以严谨方式表达对未知变量的信念,但精确推断在大规模下不可行,需依赖近似推断。当应用于深度神经网络时,贝叶斯方法面临两大瓶颈:先验设定与近似质量。本文第3、4章探讨如何利用DSM架构本身为概率模型中的先验或近似设计提供指导。首先,基于注意力机制与稀疏高斯过程的相似性,提出适配Transformer的近似贝叶斯推断方法;其次,利用HiPPO(高阶多项式投影算子)的长程记忆能力,构建高斯过程的跨域诱导点,在在线学习中成功记忆历史信息。除预测任务外,由一系列潜变量构成的序列生成模型在深度生成模型中日益流行。受扩散模型中潜变量显式自监督信号启发,第5章探索为其他生成模型引入对潜变量序列的自监督,研究其理想概率结构。总体而言,本文借助DSM的归纳偏置设计概率推断或结构,弥合了DSMs与概率模型之间的鸿沟,实现双向增强。
原文摘要 · Abstract (English)
Despite exceptional predictive performance of Deep sequence models (DSMs), the main concern of their deployment centers around the lack of uncertainty awareness. In contrast, probabilistic models quantify the uncertainty associated with unobserved variables with rules of probability. Notably, Bayesian methods leverage Bayes' rule to express our belief of unobserved variables in a principled way. Since exact Bayesian inference is computationally infeasible at scale, approximate inference is required in practice. Two major bottlenecks of Bayesian methods, especially when applied in deep neural networks, are prior specification and approximation quality. In Chapter 3 & 4, we investigate how the architectures of DSMs themselves can be informative for the design of priors or approximations in probabilistic models. We first develop an approximate Bayesian inference method tailored to the Transformer based on the similarity between attention and sparse Gaussian process. Next, we exploit the long-range memory preservation capability of HiPPOs (High-order Polynomial Projection Operators) to construct an interdomain inducing point for Gaussian process, which successfully memorizes the history in online learning. In addition to the progress of DSMs in predictive tasks, sequential generative models consisting of a sequence of latent variables are popularized in the domain of deep generative models. Inspired by the explicit self-supervised signals for these latent variables in diffusion models, in Chapter 5, we explore the possibility of improving other generative models with self-supervision for their sequential latent states, and investigate desired probabilistic structures over them. Overall, this thesis leverages inductive biases in DSMs to design probabilistic inference or structure, which bridges the gap between DSMs and probabilistic models, leading to mutually reinforced improvement.
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