Remove trainer.fit return value [2/n]

[carmocca]

What does this PR do?

See title.

Returning 1 is a legacy behavior. It does not serve any purpose as an exception would be raised if it failed.

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[codecov]

Codecov Report

Merging #7237 (74b6a37) into master (bdc4272) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #7237   +/-   ##
======================================
- Coverage      91%     91%   -0%     
======================================
  Files         199     199           
  Lines       12797   12796    -1     
======================================
- Hits        11699   11698    -1     
  Misses       1098    1098           

[Borda]

nice :]

[tchaton]

LGTM !