Continuous normalizing flows via flow matching

[janfb]

It would be great to have FMPE implemented in sbi.

There are implementations available that we could depend on, e.g.,

• dingo: https://github.com/dingo-gw/dingo/blob/FMPE/dingo/core/posterior_models/flow_matching.py
• lampe via zuko: https://lampe.readthedocs.io/en/stable/tutorials/fmpe.html

To Do:

• check how we could integrate existing implementations into sbi
• or implement from scratch
• add CNF via flow matching as density estimator
• add FMPE as inference method

[turnmanh]

I'd like to have a look at this.

[bkmi]

I am also up for it, but happy to work on something else if it's better as a one person job.

[jsvetter]

This is heavily related to #962! So it would probably be good to coordinate some aspects :)