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 69%. Comparing base (76c502b) to head (5fefa58).
Report is 2 commits behind head on master.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #2786   +/-   ##
======================================
- Coverage      69%     69%   -0%     
======================================
  Files         342     342           
  Lines       18699   18712   +13     
======================================
+ Hits        12910   12912    +2     
- Misses       5789    5800   +11     

[SkafteNicki]

@Borda can you help me with the failing docs. It fails with error:
/home/runner/work/torchmetrics/torchmetrics/src/torchmetrics/metric.py:docstring of torchmetrics.metric.Metric:6: WARNING: Explicit markup ends without a blank line; unexpected unindent.
I know it complain about missing blank line but I feel like I tried everything at this point.

[Borda]

still:

/home/jirka/Workingspace/pt-metrics/src/torchmetrics/metric.py:docstring of torchmetrics.metric.Metric:6: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/jirka/Workingspace/pt-metrics/src/torchmetrics/metric.py:docstring of torchmetrics.metric.Metric:7: WARNING: Explicit markup ends without a blank line; unexpected unindent.

[lantiga]

Awesome! Might be worth creating an example on how to use TorchMetrics within a 2D or 3D parallel DTensor-based codebase in the future.