MrVI tutorial for 1K+ samples

[PierreBoyeau]

We need to add a tutorial to guide the usage of MrVI for large single-cell RNA-seq datasets containing 1K+ samples.
In particular, we should explain what hyperparameters users should be careful about.

[ori-kron-wis]

It will be done over the Tahoe100M cells in the mrVI pyTorch version