Feature: merge_state method

[SkafteNicki]

What does this PR do?

Fixes #2643
Seems most of the logic is already implemented in private methods so it is really just creating a public method + testing.

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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--2786.org.readthedocs.build/en/2786/

[codecov]

Codecov Report

Attention: Patch coverage is 31.25000% with 11 lines in your changes missing coverage. Please review.

Project coverage is 70%. Comparing base (7f579eb) to head (0a668f2).

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #2786   +/-   ##
======================================
- Coverage      70%     70%   -0%     
======================================
  Files         336     336           
  Lines       18235   18248   +13     
======================================
+ Hits        12709   12711    +2     
- Misses       5526    5537   +11