Update _trainingplans.py

[canergen]

Disable adversarial training when using a single batch.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.80%. Comparing base (049e7eb) to head (ecb7a26). Report is 94 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2632      +/-   ##
==========================================
- Coverage   89.33%   88.80%   -0.54%     
==========================================
  Files         158      158              
  Lines       13276    13280       +4     
==========================================
- Hits        11860    11793      -67     
- Misses       1416     1487      +71     

Files	Coverage Δ
scvi/train/_trainingplans.py	94.15% <71.42%> (-0.39%)	:arrow_down:

... and 2 files with indirect coverage changes