Add hardsigmoid formula and fix WGAN doc + default lr

[laggui]

Changes

As discussed on discord, added at least the hard sigmoid definition. Don't think it's worth mentioning the default values for other frameworks necessarily, even the wiki page uses alpha = 0.2 (and not 1/6).

Also minor fix for WGAN doc (and improved default LR based on batch size)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.36%. Comparing base (f630b3b) to head (1e27b8d).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2706      +/-   ##
==========================================
+ Coverage   83.20%   83.36%   +0.16%     
==========================================
  Files         819      819              
  Lines      106814   106608     -206     
==========================================
  Hits        88870    88870              
+ Misses      17944    17738     -206     

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