Implement any mutli-source and/or multi-target DA method

[kachayev]

This is required for us to understand how the API "feels" when working with multi-source multi-target settings. It's also crucial for properly covering the functionality of selectors (note that as of now we only have basic Shared selector).

[YanisLalou]

Do you have any non-deep methods in mind to implement?

[rflamary]

This one is a classic
https://ojs.aaai.org/index.php/AAAI/article/view/9542

Also using Linear Mapping and Bures (non convolutional) barycenter. one can _implement the paper from Théo
https://arxiv.org/pdf/2305.18831.pdf
@tgnassou did you want to do it or do you prefer Yanis to have a look?