Change trainer.should_stop to not stop in between an epoch and run until min_steps/min_epochs only

[rohitgr7]

What does this PR do?

Fixes #13789

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cc @justusschock @awaelchli @rohitgr7 @Borda @carmocca @ananthsub @ninginthecloud @akihironitta

[codecov]

Codecov Report

Merging #13890 (af15229) into master (0e30e4a) will increase coverage by 15%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #13890     +/-   ##
=========================================
+ Coverage      61%      76%    +15%     
=========================================
  Files         332      332             
  Lines       26849    26902     +53     
=========================================
+ Hits        16413    20438   +4025     
+ Misses      10436     6464   -3972     

[awaelchli]

Initial comments, currently studying the loop changes and tests