add VELOVI preprocess methods

[martinkim0]

• adds min_max_scale, filter_genes_r2, and velovi_preprocess_recipe to preprocessing.utils

to-do:

• add tests
• add get_permutation_scores

[martinkim0]

Hi @WeilerP, this is the PR based on our discussion in https://github.com/scverse/scvi-tools/pull/2611. I wanted to get your thoughts on the following:

• where should I add get_permutation_scores?
• under what section in api.rst should velovi_preprocess_recipe be in?

cc @adamgayoso