rename old Trainer.train_loop -> Trainer.fit_loop

[awaelchli]

What does this PR do?

Part of #7938

Resolves a pending todo for renaming the remaining occurrences of the train_loop property which refers to the old training loop construct.

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[codecov]

Codecov Report

Merging #8025 (c83b96e) into master (dd340a6) will decrease coverage by 0%.
The diff coverage is 95%.

@@          Coverage Diff           @@
##           master   #8025   +/-   ##
======================================
- Coverage      92%     91%   -0%     
======================================
  Files         210     210           
  Lines       13590   13593    +3     
======================================
- Hits        12463   12428   -35     
- Misses       1127    1165   +38     

[ethanwharris]

LGTM 😃

[tchaton]

LGMT !