Docs on memory management

[SkafteNicki]

What does this PR do?

Fixes #1949
Add a section about good memory management advice.
Also adds a new public attribute called metric_state such that users should be able to always investigate what is currently stored by the metric.

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.

Did you have fun?

Make sure you had fun coding 🙃

---

📚 Documentation preview 📚: https://torchmetrics--2006.org.readthedocs.build/en/2006/

[codecov]

Codecov Report

Merging #2006 (b68e5b4) into master (a3eebd7) will increase coverage by 0%.
Report is 1 commits behind head on master.
The diff coverage is 100%.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #2006   +/-   ##
======================================
  Coverage      87%     87%           
======================================
  Files         270     270           
  Lines       15581   15584    +3     
======================================
+ Hits        13483   13486    +3     
  Misses       2098    2098           

[Yann-CV]

Thanks for this @SkafteNicki !