[doc] Improve Manual Optimization Example

[tchaton]

What does this PR do?

This PR provides a complete GAN example with manual optimization as requested by some users.

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[codecov]

Codecov Report

Merging #6294 (57539ec) into master (49c579f) will decrease coverage by 2%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #6294    +/-   ##
=======================================
- Coverage      93%     91%    -2%     
=======================================
  Files         160     161     +1     
  Lines       11415   11457    +42     
=======================================
- Hits        10644   10416   -228     
- Misses        771    1041   +270