Restrict setup methods to accept a single model

[awaelchli]

What does this PR do?

#9994 added a setup method setup_models_and_optimizers and this PR updates this to accept only a single model.
Follow up to the discussion in #10028 (comment) @four4fish

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[codecov]

Codecov Report

Merging #10064 (b4e5ab4) into master (e94dcf6) will increase coverage by 50%.
The diff coverage is 67%.

❗ Current head b4e5ab4 differs from pull request most recent head 8e76000. Consider uploading reports for the commit 8e76000 to get more accurate results

@@            Coverage Diff            @@
##           master   #10064     +/-   ##
=========================================
+ Coverage      43%      92%    +50%     
=========================================
  Files         182      182             
  Lines       16109    16107      -2     
=========================================
+ Hits         6874    14892   +8018     
+ Misses       9235     1215   -8020