Self-attention Mechanism at the Token Level: Gradient Analysis and Algorithm Optimization

The self-attention mechanism is a feature processing mechanism for structured data in deep learning models. It has been widely used in transformer-based deep learning models and has demonstrated superior performance in various fields, such as machine translation, speech recognition, text-to-text conversion, and computer vision. The self-attention mechanism mainly focuses on the surface structure of structured data, but it also involves attention between basic data units and self-attention of basic data units in the deeper structure of the data. In this paper, we investigate the forward attention flow and backward gradient flow in the self-attention module of the transformer model based on the sequence-to-sequence data structure used in machine translation tasks. We found that this combination produces a "gradient distortion" phenomenon at the token level of basic data units. We consider this phenomenon a defect and propose a series of solutions to address it theoretically. Furthermore, we conduct experiments and select the most robust solution as the Unevenness-Reduced Self-Attention (URSA) module, which replaces the original self-attention module. The experimental results demonstrate that the "gradient distortion" phenomenon exists both theoretically and numerically, and the URSA module enables the self-attention mechanism to achieve consistent, stable, and effective optimization across different models, tasks, corpora, and evaluation metrics. The URSA module is both simple and highly portable.