Add documentation for ways to access all batch outputs for on_train_epoch_end hook

[ananthsub]

What does this PR do?

Follow up to #7338 and #7339 to add documentation on how to best access the epoch output at the end of training, given that the outputs argument is now deprecated.

cc @Borda @awaelchli

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[codecov]

Codecov Report

Merging #7389 (5984247) into master (fbcd63a) will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #7389   +/-   ##
======================================
- Coverage      88%     87%   -0%     
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  Files         200     200           
  Lines       12985   12982    -3     
======================================
- Hits        11397   11357   -40     
- Misses       1588    1625   +37     

[awaelchli]

THANKS! :)))