test: add a test file for model instantiation

[jung235]

Add test code in tests/models.

[codecov]

Codecov Report

Attention: Patch coverage is 91.42857% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.70%. Comparing base (3c4e81d) to head (4ba7cd7).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #12       +/-   ##
===========================================
+ Coverage   53.80%   78.70%   +24.89%     
===========================================
  Files          34       35        +1     
  Lines        1221     1235       +14     
===========================================
+ Hits          657      972      +315     
+ Misses        564      263      -301     

Files	Coverage Δ
pydiffuser/models/core/base.py	97.29% <100.00%> (+13.66%)	⬆️
tests/conftest.py	100.00% <100.00%> (ø)
tests/models/test_models.py	100.00% <100.00%> (ø)
pydiffuser/utils/config.py	75.55% <50.00%> (+1.13%)	⬆️
pydiffuser/utils/jitted.py	81.08% <50.00%> (+19.54%)	⬆️

... and 12 files with indirect coverage changes