Fix reference issues during epoch end result collection

[carmocca]

What does this PR do?

Fixes 2 issues:

• The _process_training_step_output method was modifying the training step output, which could be problematic if the user saved it in training_batch_end.
• The _prepare_outputs function was modifying the current ResultCollection.extra dictionary.
• The _track_epoch_end_reduce_metrics method wasn't collapsing the sequence if only one element when it was a ResultCollection. This was probably a remnant from the Result/dict return days.

The added test covers both

Found while debugging #8613

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

None

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[codecov]

Codecov Report

Merging #8621 (df11b49) into master (c99e2fe) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #8621   +/-   ##
======================================
  Coverage      93%     93%           
======================================
  Files         168     168           
  Lines       13951   13951           
======================================
+ Hits        12909   12911    +2     
+ Misses       1042    1040    -2     

[tchaton]

LGTM !