fix collecting training_step outputs

[awaelchli]

What does this PR do?

Fixes #8603

With git bisect found that #7891 had introduced the issue by removing a "copy" of the training step output here.

Tests fail on master, reproducing the issue.

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

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Did you have fun?

not this time 😢

[codecov]

Codecov Report

Merging #8613 (80471e7) into master (5789e9f) will increase coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #8613    +/-   ##
=======================================
+ Coverage      88%     93%    +4%     
=======================================
  Files         168     168            
  Lines       13968   13969     +1     
=======================================
+ Hits        12337   12932   +595     
+ Misses       1631    1037   -594     

[ethanwharris]

LGTM 😃