  Diffusion transformers (DiTs) have exhibited remarkable performance in visual
generation tasks, such as generating realistic images or videos based on
textual instructions. However, larger model sizes and multi-frame processing
for video generation lead to increased computational and memory costs, posing
challenges for practical deployment on edge devices. Post-Training Quantization
(PTQ) is an effective method for reducing memory costs and computational
complexity. When quantizing diffusion transformers, we find that applying
existing diffusion quantization methods designed for U-Net faces challenges in
preserving quality. After analyzing the major challenges for quantizing
diffusion transformers, we design an improved quantization scheme: "ViDiT-Q":
Video and Image Diffusion Transformer Quantization) to address these issues.
Furthermore, we identify highly sensitive layers and timesteps hinder
quantization for lower bit-widths. To tackle this, we improve ViDiT-Q with a
novel metric-decoupled mixed-precision quantization method (ViDiT-Q-MP). We
validate the effectiveness of ViDiT-Q across a variety of text-to-image and
video models. While baseline quantization methods fail at W8A8 and produce
unreadable content at W4A8, ViDiT-Q achieves lossless W8A8 quantization.
ViDiTQ-MP achieves W4A8 with negligible visual quality degradation, resulting
in a 2.5x memory optimization and a 1.5x latency speedup.
