Clamp variance calculation in certain image metrics

[SkafteNicki]

What does this PR do?

Fixes #2281
In certain image metrics a variance of pixel values are calculated, which should not be able to be negative. However, in practice it can output a small negative number and even the small epsilon that we add afterwards (to deal with the variance being positive but very small) does rectify this. This PR simply clamps the variance to be 0.

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• Did you write any new necessary tests?

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📚 Documentation preview 📚: https://torchmetrics--2378.org.readthedocs.build/en/2378/

[codecov]

Codecov Report

Merging #2378 (acf9327) into master (007a44c) will decrease coverage by 31%.
Report is 1 commits behind head on master.
The diff coverage is 100%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2378     +/-   ##
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- Coverage      69%     38%    -31%     
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  Files         305     305             
  Lines       17180   17180             
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- Hits        11840    6578   -5262     
- Misses       5340   10602   +5262