Bugfix so multilabel confusion matrix can plot for 2 or more labels

[podgorki]

What does this PR do?

Fixes #2857
Multilabel plotting failed for two labels and succeeded for greater than two. No test existed for this added one but it also requires matplotlib so that was added to the test requirements

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

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📚 Documentation preview 📚: https://torchmetrics--2858.org.readthedocs.build/en/2858/

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 69%. Comparing base (a7284e2) to head (c7b5c76).
Report is 104 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #2858    +/-   ##
=======================================
- Coverage      69%     69%    -0%     
=======================================
  Files         346     332    -14     
  Lines       19133   18957   -176     
=======================================
- Hits        13229   13054   -175     
+ Misses       5904    5903     -1     

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[lantiga]

Good catch