deprecate `SaveBestState`

[martinkim0]

Users should now use scvi.train.SaveCheckpoint or the enable_checkpointing Lightning arg instead.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.81%. Comparing base (049e7eb) to head (2dd068c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2567      +/-   ##
==========================================
- Coverage   89.31%   88.81%   -0.50%     
==========================================
  Files         158      158              
  Lines       13276    13281       +5     
==========================================
- Hits        11857    11796      -61     
- Misses       1419     1485      +66     

Files	Coverage Δ
scvi/model/_multivi.py	73.52% <100.00%> (+0.09%)	:arrow_up:
scvi/model/_peakvi.py	89.34% <100.00%> (+0.26%)	:arrow_up:
scvi/train/_callbacks.py	87.66% <100.00%> (+0.08%)	:arrow_up:

... and 1 file with indirect coverage changes

[martinkim0]

closing in favor of #2673