Introduce PrecisionPlugin.forward_context()

[awaelchli]

What does this PR do?

Adds a new method forward_context and simplifies duplicated implementations across inherited plugins for train_step_context, etc.

Part of #9987

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

No.

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Did you have fun?

I made sure I had fun coding 🙃

[tchaton]

LGTM !

[codecov]

Codecov Report

Merging #9988 (7fe463e) into master (8c76cf5) will increase coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #9988    +/-   ##
=======================================
+ Coverage      89%     93%    +4%     
=======================================
  Files         179     179            
  Lines       15830   15804    -26     
=======================================
+ Hits        14056   14642   +586     
+ Misses       1774    1162   -612