Add `weighted_knn_trainer`/`weighted_knn_transfer` utilities from scArches

[seohyonkim]

Hello!

Is your feature request related to a problem? Please describe.
I am migrating a workflow from scArches to scvi-tools for the sc-best-practice. Most functionality has a clear equivalent in scvi-tools, but scarches.utils.weighted_knn_trainer and weighted_knn_transfer have no counterpart.

Describe the solution you'd like
Would it be possible to add the functions to e.g. scvi.tools? Thank you so much!

[ori-kron-wis]

Yes, however, this is not in our plan currently.
Would you like to contribute a pull request for that?
Thank you

[seohyonkim]

yes sure! I'll ping you in the PR once I have it ready. Thank you!