Fix trainer.fit_loop.split_idx reference

[carmocca]

What does this PR do?

The trainer.fit_loop.split_idx was pointing to the wrong attribute. So it always returned None

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

None

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[codecov]

Codecov Report

Merging #8601 (032e093) into master (aadd2a9) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #8601   +/-   ##
======================================
- Coverage      92%     92%   -0%     
======================================
  Files         218     218           
  Lines       14407   14406    -1     
======================================
- Hits        13305   13304    -1     
  Misses       1102    1102           

[awaelchli]

@carmocca thanks, great fix. We need to include it into 1.4.x right? I'll swap the milestone.

[tchaton]

LGTM !