Flush experiment in tensorboard logger on finalize

[ananthsub]

What does this PR do?

#2126 removed flush from save() as it was excessive in tensorboard
Here we flush in finalize to gather metrics that may not have been flushed during the async writing in order to give us better coverage

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[codecov]

Codecov Report

Merging #3920 into master will increase coverage by 5%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #3920    +/-   ##
=======================================
+ Coverage      82%     86%    +5%     
=======================================
  Files         113     113            
  Lines        8479    8520    +41     
=======================================
+ Hits         6929    7355   +426     
+ Misses       1550    1165   -385