Patch SCANVI classifier logits bug

[martinkim0]

As pointed out in offline discussions, SCANVI currently has a critical bug in its classifier code where the output of the classifier is incorrectly treated as logits (for example). By default, the classifier outputs probabilities as seen here. The effect is unknown but likely undesirable.

• Add tests for backwards compat loading and training
• Add release note
• Run small benchmark with tutorial data
• Run benchmark, possibly with OpenProblems

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (ed768ab) 89.29% compared to head (8f0ba22) 89.30%.

Files	Patch %	Lines
scvi/module/_scanvae.py	93.33%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2301   +/-   ##
=======================================
  Coverage   89.29%   89.30%           
=======================================
  Files         145      145           
  Lines       11872    11889   +17     
=======================================
+ Hits        10601    10617   +16     
- Misses       1271     1272    +1     

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