Add float target support to class & functional NDCG

[alex-senov]

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?

Fixes #436

PR review

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[pep8speaks]

Hello @obus! Thanks for updating this PR.

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

Comment last updated at 2021-08-11 07:06:41 UTC

[codecov]

Codecov Report

Merging #437 (a91e08f) into master (484477a) will decrease coverage by 0.02%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master     #437      +/-   ##
==========================================
- Coverage   95.98%   95.95%   -0.03%     
==========================================
  Files         129      129              
  Lines        4180     4178       -2     
==========================================
- Hits         4012     4009       -3     
- Misses        168      169       +1     

Flag	Coverage Δ
Linux	74.79% <87.50%> (+0.05%)	⬆️
Windows	74.79% <87.50%> (+0.05%)	⬆️
cpu	95.95% <87.50%> (-0.03%)	⬇️
macOS	95.95% <87.50%> (-0.03%)	⬇️
pytest	95.95% <87.50%> (-0.03%)	⬇️
python3.6	95.21% <87.50%> (-0.03%)	⬇️
python3.8	95.95% <87.50%> (-0.03%)	⬇️
python3.9	95.88% <87.50%> (-0.03%)	⬇️
torch1.3.1	95.21% <87.50%> (-0.03%)	⬇️
torch1.4.0	95.28% <87.50%> (-0.03%)	⬇️
torch1.9.0	95.88% <87.50%> (-0.03%)	⬇️

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

Impacted Files	Coverage Δ
torchmetrics/retrieval/retrieval_ndcg.py	100.00% <ø> (ø)
torchmetrics/utilities/checks.py	91.32% <87.50%> (-0.68%)	⬇️

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[SkafteNicki]

Small comments else looks good :]
Remember to add note in the changelog