Add current_score to ModelCheckpoint.on_save_checkpoint

[carmocca]

What does this PR do?

Fixes #4712

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[codecov]

Codecov Report

Merging #4721 (8a8ef37) into master (ece09f0) will increase coverage by 3%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #4721    +/-   ##
=======================================
+ Coverage      90%     93%    +3%     
=======================================
  Files         117     117            
  Lines        8937    8939     +2     
=======================================
+ Hits         8056    8306   +250     
+ Misses        881     633   -248     

[tchaton]

Good idea !