SSN: Learning Sparse Switchable Normalization via SparsestMax

Normalization method deals with parameters training of convolution neural networks (CNNs) in which there are often multiple convolution layers. Despite the fact that layers in CNN are not homogeneous in the role they play at representing a prediction function, existing works often employ identical normalizer in different layers, making performance away from idealism. To tackle this problem and further boost performance, a recently-proposed switchable normalization (SN) provides a new perspective for deep learning: it learns to select different normalizers for different convolution layers of a ConvNet. However, SN uses softmax function to learn importance ratios to combine normalizers, not only leading to redundant computations compared to a single normalizer but also making model less interpretable. This work addresses this issue by presenting sparse switchable normalization (SSN) where the importance ratios are constrained to be sparse. Unlike ℓ _1 and ℓ _0 regularizations that impose difficulties in tuning layer-wise regularization coefficients, we turn this sparse-constrained optimization problem into feed-forward computation by proposing SparsestMax, which is a sparse version of softmax. SSN has several appealing properties. (1) It inherits all benefits from SN such as applicability in various tasks and robustness to a wide range of batch sizes. (2) It is guaranteed to select only one normalizer for each normalization layer, avoiding redundant computations and improving interpretability of normalizer selection. (3) SSN can be transferred to various tasks in an end-to-end manner. Extensive experiments show that SSN outperforms its counterparts on various challenging benchmarks such as ImageNet, COCO, Cityscapes, ADE20K, Kinetics and MegaFace. Models and code are available at https://github.com/switchablenorms/Sparse_SwitchNorm .