Add some probabilistic modeling approaches / libs

[grisaitis]

• Compositional Perturbation Autoencoder (CPA): https://github.com/facebookresearch/CPA
• trVAE: https://github.com/theislab/trVAE
  • and its successor (?), scarches: https://github.com/theislab/scarches
    • https://scarches.readthedocs.io/en/latest/

cc @M0hammadL (cool talk at MIA today!)

[M0hammadL]

thanks, here is the missing ones :
https://github.com/theislab/scgen