Model best state

[adamgayoso]

we used to save the best state of the model before pytorch lit rebasing. Here's a simple callback that does just this.

[codecov]

Codecov Report

Merging #887 (156287a) into master (32e12bb) will decrease coverage by 0.14%.
The diff coverage is 72.91%.

@@            Coverage Diff             @@
##           master     #887      +/-   ##
==========================================
- Coverage   89.46%   89.31%   -0.15%     
==========================================
  Files          70       70              
  Lines        5296     5344      +48     
==========================================
+ Hits         4738     4773      +35     
- Misses        558      571      +13     

Impacted Files	Coverage Δ
scvi/lightning/_callbacks.py	76.36% <72.91%> (-23.64%)	⬇️

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