Fix: JaccardIndex multi-label compute

[KeVoyer1]

What does this PR do?

The shape of the matrix in multi-label mode is (nb_classes, 2, 2) and currently the compute function needs a shape of (nb_classes, nb_classes). Easy fix by a for loop in the jaccardindex compute

Before submitting

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• Did you make sure to update the docs?
• Did you write any new necessary tests?

[codecov]

Codecov Report

Merging #1125 (862ef09) into master (0d8b1b2) will decrease coverage by 42%.
The diff coverage is 67%.

@@           Coverage Diff            @@
##           master   #1125     +/-   ##
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- Coverage      94%     52%    -42%     
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  Files         182     182             
  Lines        8141    8143      +2     
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- Hits         7648    4206   -3442     
- Misses        493    3937   +3444