Another SPDnet implementation

This repository contains an alternative implementation of SPDnet from:

• Davoudi
• torchspdnet

Main differences are:

• Extensive relying on torch.einsum for increased computational speed
• Grad of BiMap coded with einsum rather than using autograd
• Works for any number of batch dimensions (rather than 1 for Davoudi and 2 for torchspdnet)
• bias term on the eigenvalues in the reEig layers ?