save last model after saving top_k when save_last=True

[awaelchli]

What does this PR do?

Fixes #2680

The checkpoint named "last.ckpt" should be saved last, literally, after saving all the top_k, because these will update the state for best_model_score and best_model_path, and if we don't save last these infos won't land in the checkpoint.

Note: Test fails on master

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[codecov]

Codecov Report

Merging #2881 into master will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #2881    +/-   ##
=======================================
- Coverage      90%     86%    -4%     
=======================================
  Files          79      79            
  Lines        7259    7229    -30     
=======================================
- Hits         6524    6217   -307     
- Misses        735    1012   +277