Convert classification wrapper to metrics

[SkafteNicki]

What does this PR do?

Fixes #1953
Make sure that even the task wrapper classes are subclasses of Metric (to make sure it works with lightning CLI, see issue).

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• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
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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--1963.org.readthedocs.build/en/1963/

[codecov]

Codecov Report

Merging #1963 (d0fec3f) into master (ea17ffb) will decrease coverage by 33%.
The diff coverage is 79%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1963     +/-   ##
========================================
- Coverage      83%     51%    -33%     
========================================
  Files         265     266      +1     
  Lines       15143   15177     +34     
========================================
- Hits        12595    7684   -4911     
- Misses       2548    7493   +4945