Add register_new_fields method to manager

[adamgayoso]

This allows updating an instance of an AnnDataManager with new fields (assuming it has already been registered with an AnnData object).

We will use this to help put models in latent mode, in which latent dists are cached in anndata and used in forward passes instead of encoding.

[codecov]

Codecov Report

Base: 91.27% // Head: 91.23% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (f9eb5b4) compared to base (9ff329e).
Patch coverage: 95.65% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1689      +/-   ##
==========================================
- Coverage   91.27%   91.23%   -0.04%     
==========================================
  Files         116      111       -5     
  Lines        9275     9245      -30     
==========================================
- Hits         8466     8435      -31     
- Misses        809      810       +1     

Impacted Files	Coverage Δ
scvi/data/_manager.py	98.19% <95.65%> (-0.51%)	⬇️
scvi/__init__.py
scvi/utils/__init__.py
scvi/train/__init__.py
scvi/_utils.py
scvi/_constants.py

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