SimpleGenerativeModel: improve base model setup

[ArneBinder]

TODO:

• [ ] implement from_config path considering is_from_pretrained as implemented here EDIT: no, some models (e.g. bartAsPointerNetwork) do not seem to have from_config()
• increase test coverage
• add PR description

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.50%. Comparing base (afcf7d4) to head (6c6a835).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #153   +/-   ##
=======================================
  Coverage   95.49%   95.50%           
=======================================
  Files          61       61           
  Lines        5195     5203    +8     
=======================================
+ Hits         4961     4969    +8     
  Misses        234      234           

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