chore(test): add pre batch key condscvi save

[martinkim0]

adds a basic CondSCVI save file for backwards compatibility testing in #2626. generated as follows:

adata = synthetic_iid()
CondSCVI.setup_anndata(adata, "labels")
model = CondSCVI(adata)
model.train(max_epochs=1)
model.save(".../tests/test_data/condscvi_pre_batch")

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.69%. Comparing base (59cf360) to head (469684c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2629      +/-   ##
==========================================
- Coverage   89.20%   88.69%   -0.52%     
==========================================
  Files         159      159              
  Lines       13308    13308              
==========================================
- Hits        11872    11803      -69     
- Misses       1436     1505      +69     

see 2 files with indirect coverage changes