solo updates

[njbernstein]

Adds early stopping by default to solo train

Adds min_lr option SCVI training

Closes #932

[codecov]

Codecov Report

Merging #933 (f01d71c) into master (f2c42c6) will increase coverage by 0.03%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master     #933      +/-   ##
==========================================
+ Coverage   89.44%   89.47%   +0.03%     
==========================================
  Files          77       79       +2     
  Lines        5863     5891      +28     
==========================================
+ Hits         5244     5271      +27     
- Misses        619      620       +1     

Impacted Files	Coverage Δ
scvi/external/solo/_model.py	95.87% <85.71%> (-0.80%)	⬇️
scvi/lightning/_trainingplans.py	94.60% <100.00%> (+0.02%)	⬆️
scvi/__init__.py	100.00% <0.00%> (ø)
scvi/_utils.py	100.00% <0.00%> (ø)

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