Bugfix for classwise wrapper compute logic

[SkafteNicki]

What does this PR do?

Fixes #1220
Some of the logic in _wrap_update and _wrap_compute was messing with the compute method in the ClasswiseWrapper. This PR makes them empty wrappers for that metric and also adds relevant testing.

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• Did you make sure to update the docs?
• Did you write any new necessary tests?

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[codecov]

Codecov Report

Merging #1225 (c6b2e39) into master (3fd3dc4) will decrease coverage by 48%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #1225     +/-   ##
========================================
- Coverage      86%     37%    -48%     
========================================
  Files         191     191             
  Lines       11319   11325      +6     
========================================
- Hits         9699    4235   -5464     
- Misses       1620    7090   +5470