Newmetric: Jensen-Shannon Divergence

[SkafteNicki]

What does this PR do?

Fixes #2947

Before submitting

• Was this discussed/agreed 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?

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📚 Documentation preview 📚: https://torchmetrics--2992.org.readthedocs.build/en/2992/

[codecov]

Codecov Report

Attention: Patch coverage is 84.21053% with 12 lines in your changes missing coverage. Please review.

Project coverage is 70%. Comparing base (d6a1ad2) to head (1e4df31).
Report is 1 commits behind head on master.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #2992   +/-   ##
======================================
  Coverage      70%     70%           
======================================
  Files         350     352    +2     
  Lines       19538   19615   +77     
======================================
+ Hits        13589   13658   +69     
- Misses       5949    5957    +8     

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