[Doc] Fix on_train_batch_end description

[amogkam]

What does this PR do?

I believe the outputs arg to on_train_batch_end hook is the result of training_step_end(training_step(x)) not validation_step_end(validation_step(x)). This PR fixes this in the docstring.

Fixes #4329

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[codecov]

Codecov Report

Merging #4330 into master will increase coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #4330   +/-   ##
======================================
  Coverage      93%     93%           
======================================
  Files         111     111           
  Lines        8085    8063   -22     
======================================
- Hits         7486    7472   -14     
+ Misses        599     591    -8     

[rohitgr7]

pls wait. a few more are there I think

[rohitgr7]

[skip ci] doesn't work anymore? This PR seems to be running gpu tests.

[awaelchli]

[skip ci] doesn't work anymore? This PR seems to be running gpu tests.

I think it is because I added it retrospectively?