Update signature of LightningModule.training_step_end

[awaelchli]

What does this PR do?

Addresses #6404 (comment)

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Did you have fun?

I made sure I had fun coding 🙃

Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

[codecov]

Codecov Report

Merging #10094 (79a918a) into master (3a4e997) will increase coverage by 4%.
The diff coverage is 100%.

❗ Current head 79a918a differs from pull request most recent head 086f918. Consider uploading reports for the commit 086f918 to get more accurate results

@@           Coverage Diff            @@
##           master   #10094    +/-   ##
========================================
+ Coverage      89%      92%    +4%     
========================================
  Files         180      182     +2     
  Lines       16146    16157    +11     
========================================
+ Hits        14317    14942   +625     
+ Misses       1829     1215   -614     

[tchaton]

LGTM !