Dimple: Discrete Diffusion Multimodal Large Language Model with Parallel
  Decoding
Authors: name: Runpeng Yu, name: Xinyin Ma, name: Xinchao Wang

In this work, we propose Dimple, the first Discrete Diffusion Multimodal
Large Language Model (DMLLM). We observe that training with a purely discrete
diffusion approach leads to significant training instability, suboptimal
performance, and severe length bias issues. To address these challenges, we
design a novel training paradigm that combines an initial autoregressive phase
with a subsequent diffusion phase. This approach yields the Dimple-7B model,
trained on the same dataset and using a similar training pipeline as
LLaVA-NEXT. Dimple-7B ultimately surpasses LLaVA-NEXT in performance by 3.9%,
demonstrating that DMLLM can achieve performance comparable to that of
autoregressive models. To improve inference efficiency, we propose a decoding
strategy termed confident decoding, which dynamically adjusts the number of
tokens generated at each step, significantly reducing the number of generation
iterations. In autoregressive models, the number of forward iterations during
generation equals the response length. With confident decoding, however, the
number of iterations needed by Dimple is even only text{response
length}{3}. We also re-implement the prefilling technique in autoregressive
models and demonstrate that it does not significantly impact performance on
most benchmark evaluations, while offering a speedup of 1.5x to 7x.
Additionally, we explore Dimple's capability to precisely control its response
using structure priors. These priors enable structured responses in a manner
distinct from instruction-based or chain-of-thought prompting, and allow
fine-grained control over response format and length, which is difficult to
achieve in autoregressive models. Overall, this work validates the feasibility
and advantages of DMLLM and enhances its inference efficiency and
controllability. Code and models are available at
https://github.com/yu-rp/Dimple.