docs: Remove optimizer from manual_backward args

[akihironitta]

What does this PR do?

Removes optimizer arg from manual_backward in the docs.

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[codecov]

Codecov Report

Merging #6267 (111295a) into master (352e8f0) will decrease coverage by 2%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #6267    +/-   ##
=======================================
- Coverage      93%     91%    -2%     
=======================================
  Files         159     159            
  Lines       11362   11362            
=======================================
- Hits        10605   10335   -270     
- Misses        757    1027   +270     

[tchaton]

LGTM !