scvi: normalizing flow prior

[sjfleming]

Try
https://zuko.readthedocs.io/stable/index.html

which I learned about from Pyro.

It is a good bet that making the prior in scVI more expressive could lead to improvements in the fidelity of reconstructions, and in the expressivity of the posterior.

It's probably not that hard to implement. Harder to know if it's helping and how much.