Transfer veloVI to external

[martinkim0]

For users transferring from the original velovi package, the public API has been kept identical in the following locations:

• velovi.VELOVI -> scvi.external.VELOVI
• velovi.VELOVAE -> scvi.external.velovi.VELOVAE
• velovi.get_permutation_scores -> scvi.external.velovi.get_permutation_scores
• velovi.preprocess_data -> scvi.external.velovi.preprocess_data

This will be available starting with scvi-tools v1.2.0.