Allow target nDCG metric to be integer larger than 1

[SkafteNicki]

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• Did you write any new necessary tests?

What does this PR do?

Fixes #343
Allows target in nDCG retrieval metric to be an integer larger than 1

PR review

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[codecov]

Codecov Report

Merging #349 (28cf726) into master (3f02ba2) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #349      +/-   ##
==========================================
- Coverage   96.44%   96.39%   -0.06%     
==========================================
  Files         112      112              
  Lines        3657     3659       +2     
==========================================
  Hits         3527     3527              
- Misses        130      132       +2     

Flag	Coverage Δ
Linux	76.35% <100.00%> (+0.01%)	⬆️
Windows	76.35% <100.00%> (+0.01%)	⬆️
cpu	96.39% <100.00%> (+<0.01%)	⬆️
gpu	?
macOS	96.39% <100.00%> (+<0.01%)	⬆️
pytest	96.39% <100.00%> (-0.06%)	⬇️
python3.6	95.54% <100.00%> (+<0.01%)	⬆️
python3.8	96.39% <100.00%> (+<0.01%)	⬆️
python3.9	?
torch1.3.1	95.54% <100.00%> (+<0.01%)	⬆️
torch1.4.0	95.62% <100.00%> (+<0.01%)	⬆️
torch1.9.0	96.28% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
torchmetrics/retrieval/retrieval_metric.py	100.00% <100.00%> (ø)
torchmetrics/retrieval/retrieval_ndcg.py	100.00% <100.00%> (ø)
torchmetrics/functional/regression/spearman.py	93.33% <0.00%> (-4.45%)	⬇️

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