fix scANVI max_epochs bug when pretrained

[galenxing]

We have a heuristic for max_epochs if scanvi was pretrained, but the heuristic would overwrite max_epochs the user provided.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.67%. Comparing base (49ae51f) to head (9dffd10).

Files	Patch %	Lines
scvi/model/_scanvi.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1079      +/-   ##
==========================================
- Coverage   90.70%   90.67%   -0.03%     
==========================================
  Files          90       90              
  Lines        6745     6745              
==========================================
- Hits         6118     6116       -2     
- Misses        627      629       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.