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We show that conventional Low Rank Adaptation (LoRA) leads to suboptimal finetuning of models with large width (embedding dimension). This is due to the fact that adapter matrices A and B in LoRA are updated with the same learning rate. Using scaling arguments for large width networks, we demonstrate that using the same learning rate for A and B does not allow efficient feature learning. We then show that this suboptimality of LoRA can be corrected simply by setting different learning rates for the LoRA adapter matrices A and B with a well-chosen ratio. We call this proposed algorithm LoRA+. In our extensive experiments, LoRA+ improves performance (1-2 % improvements) and finetuning speed (up to ∼ 2X SpeedUp), at the same computational cost as LoRA.

LoRA+: Efficient Low Adaptation of Large Models. ICML 2024. Soufiane Hayou, Nikhil Ghosh, Bin Yu. Paper, Blog.

We show that taking the width and depth to infinity in a deep neural network with skip connections, when branches are scaled by 1/sqrt{depth} (the only nontrivial scaling), result in the same covariance structure no matter how that limit is taken. This explains why the standard infinite-width-then-depth approach provides practical insights even for networks with depth of the same order as width. We also demonstrate that the pre-activations, in this case, have Gaussian distributions which has direct applications in Bayesian deep learning. We conduct extensive simulations that show an excellent match with our theoretical findings.

Width and Depth Limits Commute in Residual Networks. ICML 2023. Soufiane Hayou, Greg Yang. Link

During the training of deep neural networks, some layers align much more with data compared to other layers (where the alignment is defined as the euclidean product of the tangent features matrix and the data labels matrix). The curve of the alignment as a function of layer index (generally) exhibits an ascent-descent pattern where the maximum is reached for some hidden layer. In this work, we provide the first explanation for this phenomenon. We introduce the Equilibrium Hypothesis which connects this alignment pattern to signal propagation in deep neural networks. Our experiments demonstrate an excellent match with the theoretical predictions.

Feature Learning and Signal Propagation in Deep Neural Networks. ICML 2022. Yizhang Lou, Chris Mingard, Yoonsoo Nam, Soufiane Hayou. Link

Regularization plays a major role in modern deep learning. From classic techniques such as L1,L2 penalties to other noise-based methods such as Dropout, regularization often yields better generalization properties by avoiding overfitting. Recently, Stochastic Depth (SD) has emerged as an alternative regularization technique for residual neural networks (ResNets) and has proven to boost the performance of ResNet on many tasks. Despite the recent success of SD, little is known about this technique from a theoretical perspective. This paper provides a hybrid analysis combining perturbation analysis and signal propagation to shed light on different regularization effects of SD. Our analysis allows us to derive principled guidelines for the choice of the survival rates used when training with SD.

Regularization in ResNet with Stochastic Depth. NeurIPS 2021. Soufiane Hayou, Fadhel Ayed. Link

Deep ResNet architectures have achieved state of the art performance on many tasks. While they solve the problem of gradient vanishing, they might suffer from gradient exploding as the depth becomes large ( Yang et al. 2017). Moreover, recent results have shown that ResNet might lose expressivity as the depth goes to infinity ( Yang et al. 2017, Hayou et al. 2019). To resolve these issues, we introduce a new class of ResNet architectures, called Stable ResNet, that have the property of stabilizing the gradient while ensuring expressivity in the infinite depth limit.

• Stable ResNet. AISTATS 2021 (Oral presentation). Soufiane Hayou, Eugenio Clerico, Bobby He, George Deligiannidis, Arnaud Doucet, Judith Rousseau. Link

Overparameterized Neural Networks (NN) display state-of-the art performance. However, there is a growing need for smaller, energy-efficient, neural networks to be able to use machine learning applications on devices with limited computational resources. A popular approach consists of using pruning techniques. While these techniques have traditionally focused on pruning pre-trained NN , recent reasearch have shown promising results when pruning at initialization. In this paper we provide a comprehensive theoretical analysis of pruning at initialization and training of sparse architectures. This allows us to propose novel principled approaches which we validate experimentally on a variety of NN architectures.

Robust Pruning at Initialization. ICLR 2021. Soufiane Hayou, Jean-Francois Ton, Arnaud Doucet, Yee Whye Teh. Link

The figures on the side represent the output of a randomly initialized neural network of depth 100 and width 100 with Tanh activation function and input space [0,1]x[0,1]. When the network is initialized on the Chaotic phase, the output function is discontinuous almost everywhere (in the limit of infinite width), in this case, two inputs that are close (in euclidean distance) have very different output values. On the other hand, an initialization known as the Edge of Chaos yields smooth regular output functions. In this paper, we give a comprehensive analysis of the Edge of Chaos initialization and show the critical role of the smoothness of the Activation function in deep neural networks training.

On the Impact of the Activation function on Deep Neural Networks Training. ICML 2019. Soufiane Hayou, Arnaud Doucet, Judith Rousseau. Link