scvi models should support being loaded without full counts

[watiss]

There is a fair amount of things we can do with a loaded model without needing access to the full adata, using only the latent representation and cell metadata (obs/obsm). For instance, we can pass the latent representation through the decoder and get high dimensional info such as denoised gene expression, likelihood params, posterior predictive, etc. Yet there is no way to do that because of the way the API is implemented where almost any paths requires a full adata, though it's not always needed. This issue is to change that.