Auroc error message

[SkafteNicki]

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

What does this PR do?

Fixes #234
Adds better error message for AUROC when evaluation multi class but does not provide the num_classes argument.

PR review

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[codecov]

Codecov Report

Merging #244 (0b42ad7) into master (078671f) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #244      +/-   ##
==========================================
- Coverage   96.82%   96.80%   -0.02%     
==========================================
  Files         184       92      -92     
  Lines        6010     3007    -3003     
==========================================
- Hits         5819     2911    -2908     
+ Misses        191       96      -95     

Flag	Coverage Δ
Linux	79.08% <50.00%> (-0.02%)	⬇️
Windows	79.08% <50.00%> (-0.02%)	⬇️
cpu	96.80% <100.00%> (+<0.01%)	⬆️
gpu	?
macOS	96.80% <100.00%> (+<0.01%)	⬆️
pytest	96.80% <100.00%> (-0.02%)	⬇️
python3.6	95.74% <100.00%> (+<0.01%)	⬆️
python3.8	96.80% <100.00%> (+<0.01%)	⬆️
python3.9	96.70% <100.00%> (+<0.01%)	⬆️
torch1.3.1	95.74% <100.00%> (+<0.01%)	⬆️
torch1.4.0	95.87% <100.00%> (+<0.01%)	⬆️
torch1.8.1	96.70% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
torchmetrics/functional/classification/auroc.py	86.56% <100.00%> (+0.41%)	⬆️
...cs/functional/regression/mean_squared_log_error.py
__w/1/s/torchmetrics/classification/stat_scores.py
__w/1/s/torchmetrics/retrieval/retrieval_ndcg.py
__w/1/s/torchmetrics/retrieval/__init__.py
.../s/torchmetrics/classification/confusion_matrix.py
__w/1/s/torchmetrics/utilities/prints.py
__w/1/s/torchmetrics/setup_tools.py
__w/1/s/torchmetrics/retrieval/retrieval_metric.py
...chmetrics/classification/precision_recall_curve.py
... and 83 more

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