fix RESpanPairClassificationTaskModule

[ArneBinder]

• fix end indices (we used one token after the marker token before)
• fix missed tokens when using partition annotations (any marker token at the beginning of a partition, i.e. sentence, was cut away)

EDIT: the fix for the decoding was outsourced to #102 which this is already rebased on

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.45%. Comparing base (a440655) to head (52a5d70).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #101   +/-   ##
=======================================
  Coverage   95.45%   95.45%           
=======================================
  Files          61       61           
  Lines        5127     5127           
=======================================
  Hits         4894     4894           
  Misses        233      233           

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