support for cartesian product maniforlds

**Is your feature request related to a problem? Please describe.**

The original Riemannian flow matching repo had nice support for product spaces (e.g. sphere x sphere, sphere x tori x euclidean) https://github.com/facebookresearch/riemannian-fm

As far as I can see this is not supported yet in this repo, but would be very nice to have.

**Describe the solution you'd like**
A set of top-level manifold types that can be initialized from a set of individual manifolds to represent the cartesian product

**Describe alternatives you've considered**
One option would be to go back to the old repo `riemannian-fm` which uses `geoopt` as its manifold library.

I would be open to try to contribute such extensions