add num_classes argument to confusion matrix

[c00k1ez]

What does this PR do?

Add num_classes argument to metrics.ConfusionMatrixand functional.confusion_matrix, update test_confusion_matrix() . Discussed this issue with @SkafteNicki in Slack.

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[c00k1ez]

@SkafteNicki Check this PR plz)

[codecov]

Codecov Report

Merging #3450 into master will increase coverage by 0%.
The diff coverage is 97%.

@@          Coverage Diff           @@
##           master   #3450   +/-   ##
======================================
  Coverage      90%     90%           
======================================
  Files         103     104    +1     
  Lines        8112    8125   +13     
======================================
+ Hits         7295    7313   +18     
+ Misses        817     812    -5     

[SkafteNicki]

LGTM. Very good job :]