Docs and tests for how to save and load metrics

[SkafteNicki]

What does this PR do?

Fixes #2223
Adds documentation and tests for how to save and load metrics.

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?

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

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Make sure you had fun coding 🙃

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📚 Documentation preview 📚: https://torchmetrics--2237.org.readthedocs.build/en/2237/

[codecov]

Codecov Report

Merging #2237 (2e6a61b) into master (0cb4c79) will decrease coverage by 48%.
Report is 1 commits behind head on master.
The diff coverage is 4%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2237     +/-   ##
========================================
- Coverage      87%     39%    -48%     
========================================
  Files         293     293             
  Lines       16426   16432      +6     
========================================
- Hits        14277    6332   -7945     
- Misses       2149   10100   +7951