[New metric] CohenKappa

[SkafteNicki]

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• 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?

Partly fixes #59.
Issue requests two new metrics. This PR implements the CohenKappa score.

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[pep8speaks]

Hello @SkafteNicki! Thanks for updating this PR.

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-03-15 14:55:23 UTC

[codecov]

Codecov Report

Merging #69 (0354fec) into master (dfc85be) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
+ Coverage   97.08%   97.15%   +0.06%     
==========================================
  Files         118       61      -57     
  Lines        3912     2003    -1909     
==========================================
- Hits         3798     1946    -1852     
+ Misses        114       57      -57     

Flag	Coverage Δ
Linux	78.77% <85.10%> (+0.15%)	⬆️
Windows	78.77% <85.10%> (+0.15%)	⬆️
cpu	97.15% <100.00%> (+0.06%)	⬆️
gpu	?
macOS	97.15% <100.00%> (+0.06%)	⬆️
pytest	97.15% <100.00%> (+0.06%)	⬆️
python3.6	97.14% <100.00%> (+0.06%)	⬆️
python3.8	97.15% <100.00%> (+0.06%)	⬆️
python3.9	97.15% <100.00%> (+0.06%)	⬆️
torch1.3.1	95.44% <100.00%> (+0.10%)	⬆️
torch1.4.0	95.60% <100.00%> (+0.10%)	⬆️
torch1.8.0	97.15% <100.00%> (+0.06%)	⬆️

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

Impacted Files	Coverage Δ
torchmetrics/__init__.py	100.00% <ø> (ø)
torchmetrics/classification/__init__.py	100.00% <100.00%> (ø)
torchmetrics/classification/cohen_kappa.py	100.00% <100.00%> (ø)
torchmetrics/functional/__init__.py	100.00% <100.00%> (ø)
torchmetrics/functional/classification/__init__.py	100.00% <100.00%> (ø)
...chmetrics/functional/classification/cohen_kappa.py	100.00% <100.00%> (ø)
__w/1/s/torchmetrics/collections.py
.../1/s/torchmetrics/functional/regression/r2score.py
.../s/torchmetrics/functional/classification/auroc.py
...trics/functional/regression/mean_absolute_error.py
... and 57 more

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[Borda]

very clean, great!