Add custom logic to each OutputResult subclass [2/2]

[carmocca]

What does this PR do?

Follow-up to #9349

• Treat everything returned by the manual optimization training_step as an "extra".
• Customize ManualResult logic.
• Tighten output validation.
• Integrate output validation into the appropriate result class.

Does your PR introduce any breaking changes? If yes, please list them.

None

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[tchaton]

LGTM !

[codecov]

Codecov Report

Merging #9424 (fc91d13) into master (5735b85) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #9424    +/-   ##
=======================================
- Coverage      93%     89%    -4%     
=======================================
  Files         180     180            
  Lines       15105   15090    -15     
=======================================
- Hits        14031   13404   -627     
- Misses       1074    1686   +612