Feature: multioutput mse

[SkafteNicki]

What does this PR do?

Fixes #1929

Before submitting

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• Did you make sure to update the docs?
• Did you write any new necessary tests?

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📚 Documentation preview 📚: https://torchmetrics--1937.org.readthedocs.build/en/1937/

[codecov]

Codecov Report

Merging #1937 (68ea8d0) into master (b22ca97) will decrease coverage by 47%.
Report is 1 commits behind head on master.
The diff coverage is 92%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1937     +/-   ##
========================================
- Coverage      86%     40%    -47%     
========================================
  Files         266     266             
  Lines       15185   15198     +13     
========================================
- Hits        13131    6028   -7103     
- Misses       2054    9170   +7116