Fix on_train_batch_start hook to end epoch early

[ydcjeff]

What does this PR do?

Fixes #3693

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[ydcjeff]

It's ready to review.

[Borda]

lgtm

[codecov]

Codecov Report

Merging #3700 into master will decrease coverage by 6%.
The diff coverage is 33%.

@@           Coverage Diff            @@
##           master   #3700     +/-   ##
========================================
- Coverage      85%     79%     -6%     
========================================
  Files         110     112      +2     
  Lines        8387   10236   +1849     
========================================
+ Hits         7170    8114    +944     
- Misses       1217    2122    +905     

[SkafteNicki]

Lgtm

[rohitgr7]

LGTM :)