Add the on_before_backward hook

[tchaton]

What does this PR do?

This PR adds on_before_backward hook and passes only the loss.

Fixes #7789

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[codecov]

Codecov Report

Merging #7865 (b47470b) into master (1ad1a89) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #7865   +/-   ##
======================================
- Coverage      92%     92%   -0%     
======================================
  Files         214     214           
  Lines       13891   13900    +9     
======================================
- Hits        12816   12812    -4     
- Misses       1075    1088   +13     

[awaelchli]

Is it not better to call on_before_backward in the training loop directly (instead of plugins) like we do for the on_after_backward? Or is there a blocker?

otherwise LGTM makes sense to include this hook