remove backward from training batch loop

[awaelchli]

What does this PR do?

Part of #7938
Follow up to #9191

Resolves a TODO after splitting automatic optimization into a separate loop class.
The backward method on the TrainingBatchLoop is no necessary anymore as it is only rerouting a call to the accelerator.

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Did you have fun?

I made sure I had fun coding 🙃

[tchaton]

LGTM !

[codecov]

Codecov Report

Merging #9265 (45061bc) into master (7535093) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #9265    +/-   ##
=======================================
- Coverage      92%     88%    -4%     
=======================================
  Files         178     178            
  Lines       14889   14885     -4     
=======================================
- Hits        13736   13118   -618     
- Misses       1153    1767   +614