Automatic Optimisation of Normalised Neural Networks
Namhoon Cho Hyo-Sang Shin
We propose automatic optimisation methods considering the geometry of matrix
manifold for the normalised parameters of neural networks. Layerwise weight
normalisation with respect to Frobenius norm is utilised to bound the Lipschitz
constant and to enhance gradient reliability so that the trained networks are
suitable for control applications. Our approach first initialises the network
and normalises the data with respect to the $\ell^{2}$-$\ell^{2}$ gain of the
initialised network. Then, the proposed algorithms take the update structure
based on the exponential map on high-dimensional spheres. Given an update
direction such as that of the negative Riemannian gradient, we propose two
different ways to determine the stepsize for descent. The first algorithm
utilises automatic differentiation of the objective function along the update
curve defined on the combined manifold of spheres. The directional second-order
derivative information can be utilised without requiring explicit construction
of the Hessian. The second algorithm utilises the majorisation-minimisation
framework via architecture-aware majorisation for neural networks. With these
new developments, the proposed methods avoid manual tuning and scheduling of
the learning rate, thus providing an automated pipeline for optimizing
normalised neural networks.