Move generative modelling into a separate package

[RaunoArike]

Currently, this package implements a variational autoencoder from scratch. This is not a necessary part of the package: instead, we could explore using GenerativeModels.jl.

[pat-alt]

Small enough overhead to not address and keep it here. If/when a solid standalone package for generative models pops up, we can easily incorporate here (much like with MLJTransform, MultivariateStats, ...)