Add optional color map parameter for confusion matrix

[randombenj]

What does this PR do?

This change allows to specify a cmap for the confusion matrix, making the matrix (IMO) more readable.
Here is an example specifying cmap="Blues":

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• Did you write any new necessary tests?

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📚 Documentation preview 📚: https://torchmetrics--2512.org.readthedocs.build/en/2512/

[codecov]

Codecov Report

Merging #2512 (1c1a955) into master (af32fd0) will decrease coverage by 0%.
The diff coverage is 29%.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #2512   +/-   ##
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- Coverage      69%     69%   -0%     
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  Files         313     313           
  Lines       17596   17598    +2     
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+ Hits        12146   12147    +1     
- Misses       5450    5451    +1