arXiv:2601.02799cs.LGcs.CL2026-01

提出新型采样方法,解决离散扩散模型生成时的编辑不稳问题。

Stratified Hazard Sampling: Minimal-Variance Event Scheduling for CTMC/DTMC Discrete Diffusion and Flow Models

  • 按累积风险分层采样,实现最小方差的编辑次数控制
  • 在保持多模态性的前提下,显著减少残余噪声与连锁替换
  • 无需训练调整,适用于多种离散扩散模型,尤其适合强约束场景

统一噪声的离散扩散与流模型(如 D3PM、SEDD、UDLM、DFM)通过多次上下文相关的词元替换非自回归地生成序列。这类模型通常建模为时变连续时间马尔可夫链(CTMC)或离散时间马尔可夫链(DTMC),并在每个离散步骤独立使用伯努利决策进行更新,导致每位置跳跃次数呈现泊松-二项分布方差,随所需编辑数增长而增大,引发典型的编辑不足(残余噪声)和过度编辑(级联替换)问题,尤其在紧缩离散化预算下更严重。相比之下,吸收态(掩码起始)模型通过限制每位置最多跳跃一次避免此不稳定性。本文提出分层风险采样(SHS),一种无需训练、可直接嵌入任意具备‘停留-替换’分解的采样器的推理原则。SHS将每词元编辑建模为由累积风险(CTMC)或累积跳跃质量(DTMC)驱动的事件,并通过分层方式放置这些事件:每个位置仅需一个随机相位,当其累积风险跨越等距阈值时即执行更新。该方法在保持期望跳跃数不变的前提下,达到无偏整数估计器中最小可能的条件方差(对固定累积质量,方差上限为1/4),且不改变每次跳跃的目标采样策略,因而保留多模态性。在统一噪声离散扩散语言模型上的实验表明,SHS能持续提升生成质量。进一步实验证明,SHS在词元级黑名单过滤下更具鲁棒性,且随着词汇约束加剧,优势越明显。

原文摘要 · Abstract (English)

Uniform-noise discrete diffusion and flow models (e.g., D3PM, SEDD, UDLM, DFM) generate sequences non-autoregressively by iteratively refining randomly initialized vocabulary tokens through multiple context-dependent replacements. These models are typically formulated as time-inhomogeneous CTMC/DTMC processes and sampled using independent Bernoulli change decisions at each discretization step. This induces Poisson-binomial variance in per-position jump counts that grows with the number of required edits, leading to the characteristic under-editing (residual noise) and over-editing (cascading substitutions) failure modes that degrade sample quality, especially under tight discretization budgets. In contrast, absorbing-state (mask-start) models avoid this instability by allowing each position to jump at most once. We propose Stratified Hazard Sampling (SHS), a training-free, drop-in, and hyperparameter-free inference principle for any sampler that admits a stay-vs.-replace decomposition. SHS models per-token edits as events driven by cumulative hazard (CTMC) or cumulative jump mass (DTMC) and places events by stratifying this cumulative quantity: with a single random phase per position, a token is updated whenever its accumulated hazard crosses unit-spaced thresholds. This preserves the expected number of jumps while achieving the minimum possible conditional variance among unbiased integer estimators (bounded by 1/4 for any fixed cumulative mass), without altering per-jump destination sampling and thus retaining multimodality. Experiments on uniform-noise discrete diffusion language models show that SHS consistently improves sample quality. We further show that SHS improves robustness under token-level blacklist filtering, with benefits increasing as lexical constraints grow more severe.

离散扩散采样优化语言模型

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