Vision Transformers (ViT) have shown their competitive advantages performance-wise compared to convolutional neural networks (CNNs) though they often come with high computational costs. To this end, previous methods explore different attention patterns by limiting a fixed number of spatially nearby tokens to accelerate the ViT's multi-head self-attention (MHSA) operations. However, such structured attention patterns limit the token-to-token connections to their spatial relevance, which disregards learned semantic connections from a full attention mask. In this work, we propose a novel approach to learn instance-dependent attention patterns, by devising a lightweight connectivity predictor module to estimate the connectivity score of each pair of tokens. Intuitively, two tokens have high connectivity scores if the features are considered relevant either spatially or semantically. As each token only attends to a small number of other tokens, the binarized connectivity masks are often very sparse by nature and therefore provide the opportunity to accelerate the network via sparse computations. Equipped with the learned unstructured attention pattern, sparse attention ViT (Sparsifiner) produces a superior Pareto-optimal trade-off between FLOPs and top-1 accuracy on ImageNet compared to token sparsity. Our method reduces 48% to 69% FLOPs of MHSA while the accuracy drop is within 0.4%. We also show that combining attention and token sparsity reduces ViT FLOPs by over 60%.

本文提出了一种基于学习的、实例相关的注意力机制来加速Vision Transformers网络，其将自注意力操作限制在空间上邻近的一组Token上，并通过轻量级的连接性预测器模块评估Token之间的连接得分来解决由结构化注意力模式引起的语义信息丧失问题，可以在保证准确率下显著减少计算量，达到更优的精度－计算复杂度平衡点，进一步结合Token稀疏机制，该方法可以将Vision Transformers网络的FLOPs降低超过60%。

Sparsifiner：学习稀疏实例相关注意力以提高视觉Transformer的效率