deprecate is_multiclass

[Borda]

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• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

ANother deprecation round...

PR review

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[codecov]

Codecov Report

Merging #194 (65d93b8) into master (4483658) will decrease coverage by 0.12%.
The diff coverage is 78.94%.

@@            Coverage Diff             @@
##           master     #194      +/-   ##
==========================================
- Coverage   96.75%   96.63%   -0.13%     
==========================================
  Files         180      180              
  Lines        5728     5760      +32     
==========================================
+ Hits         5542     5566      +24     
- Misses        186      194       +8     

Flag	Coverage Δ
Linux	79.93% <78.94%> (-0.03%)	⬇️
Windows	79.93% <78.94%> (-0.03%)	⬇️
cpu	96.63% <78.94%> (-0.13%)	⬇️
gpu	96.63% <ø> (-0.13%)	⬇️
macOS	96.63% <78.94%> (-0.13%)	⬇️
pytest	96.63% <78.94%> (-0.13%)	⬇️
python3.6	95.54% <78.94%> (-0.12%)	⬇️
python3.8	96.59% <78.94%> (-0.13%)	⬇️
python3.9	96.49% <78.94%> (-0.13%)	⬇️
torch1.3.1	95.54% <78.94%> (-0.12%)	⬇️
torch1.4.0	95.65% <78.94%> (-0.12%)	⬇️
torch1.8.1	96.49% <78.94%> (-0.13%)	⬇️

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

Impacted Files	Coverage Δ
torchmetrics/utilities/__init__.py	68.75% <50.00%> (-20.14%)	⬇️
torchmetrics/classification/precision_recall.py	100.00% <100.00%> (ø)
torchmetrics/classification/stat_scores.py	100.00% <100.00%> (ø)
...rics/functional/classification/precision_recall.py	100.00% <100.00%> (ø)
...chmetrics/functional/classification/stat_scores.py	100.00% <100.00%> (ø)
__w/1/s/torchmetrics/utilities/__init__.py	68.75% <0.00%> (-20.14%)	⬇️
__w/1/s/torchmetrics/classification/stat_scores.py	100.00% <0.00%> (ø)
.../s/torchmetrics/classification/precision_recall.py	100.00% <0.00%> (ø)
...chmetrics/functional/classification/stat_scores.py	100.00% <0.00%> (ø)
...rics/functional/classification/precision_recall.py	100.00% <0.00%> (ø)

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