lionel-lig-5887 remove heuristics in masked pooling

[liopeer]

Changes

• removes automatic detection of num_cls and makes this argument non-optionable
• removes the test for automatic inference of num_cls
• fixes an issue with the argument reduce, which was not actually propagated
• brush up the docstring while at it

Necessity

• using the heuristics was bad practice and could lead to issue when calling the function several times with different masks (e.g. different tensor output shapes from the two calls)
• user of function should be aware of the classes in the dataset anyway

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.24%. Comparing base (f6ad8c7) to head (62875c4).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1777      +/-   ##
==========================================
- Coverage   85.25%   85.24%   -0.01%     
==========================================
  Files         163      163              
  Lines        6834     6832       -2     
==========================================
- Hits         5826     5824       -2     
  Misses       1008     1008              

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[guarin]

LGTM!