Transformers with Competitive Ensembles of Independent Mechanisms
Authors: Alex Lamb, Di He, Anirudh Goyal, Guolin Ke, Chien-Feng Liao, Mirco Ravanelli, Yoshua Bengio
Keywords: transformer, mechanism, modularity, modules, independence
Abstract: An important development in deep learning from the earliest MLPs has been a move towards architectures with structural inductive biases which enable the model to keep distinct sources of information and routes of processing well-separated.  This structure is linked to the notion of independent mechanisms from the causality literature, in which a mechanism is able to retain the same processing as irrelevant aspects of the world are changed.  For example, convnets enable separation over positions, while attention-based architectures (especially Transformers) learn which combination of positions to process dynamically.  In this work we explore a way in which the Transformer architecture is deficient: it represents each position with a large monolithic hidden representation and a single set of parameters which are applied over the entire hidden representation.  This potentially throws unrelated sources of information together, and limits the Transformer's ability to capture independent mechanisms.  To address this, we propose Transformers with Independent Mechanisms (TIM), a new Transformer layer which divides the hidden representation and parameters into multiple mechanisms, which only exchange information through attention.  Additionally, we propose a competition mechanism which encourages these mechanisms to specialize over time steps, and thus be more independent.  We study TIM on a large scale BERT model, on the Image Transformer, and on speech enhancement and find evidence for semantically meaningful specialization as well as improved performance.