Remove unnecessary TrainingEpochLoop return

[carmocca]

What does this PR do?

We were returning the training_epoch_end input as the output of TrainingEpochLoop, I believe this is legacy and no longer necessary. Let's see what CI thinks

Does your PR introduce any breaking changes? If yes, please list them.

None

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[codecov]

Codecov Report

Merging #9298 (9a15f78) into master (b91747e) will increase coverage by 0%.
The diff coverage is 93%.

@@           Coverage Diff           @@
##           master   #9298    +/-   ##
=======================================
  Coverage      92%     93%            
=======================================
  Files         178     178            
  Lines       14855   15650   +795     
=======================================
+ Hits        13704   14482   +778     
- Misses       1151    1168    +17     

[tchaton]

LGTM !

[awaelchli]

looks like we are covering the edge case, hope we don't miss another one 🙈