[feat] Allow overriding optimizer_zero_grad and/or optimizer_step when using accumulate_grad_batches

[DavidMChan]

What does this PR do?

Adds the ability to override optimizer_zero_grad when using accumulate_grad_batches as discussed in #6910

Fixes #6910

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[codecov]

Codecov Report

Merging #7980 (2a76b31) into master (bc2c2db) will decrease coverage by 5%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #7980    +/-   ##
=======================================
- Coverage      92%     87%    -5%     
=======================================
  Files         207     207            
  Lines       13374   13373     -1     
=======================================
- Hits        12246   11603   -643     
- Misses       1128    1770   +642     

[carmocca]

Thanks!