Fix wrong accumulation with average='none'

[kpoeppel]

What does this PR do?

Fixes #1045, simply copying tensors before they are mutated in-place.

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• Did you make sure to update the docs?
• Did you write any new necessary tests?
  -> Feel free to add the test mentioned in Accuracy, Precision, Recall, Specificity wrong accumulation with average='none' inside Wrapper. #1045

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[Borda]

pls add the test mentioned in #1045

[codecov]

Codecov Report

Merging #1046 (c2b49c9) into master (fa44471) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1046   +/-   ##
======================================
- Coverage      95%     95%   -0%     
======================================
  Files         181     181           
  Lines        8004    8004           
======================================
- Hits         7582    7581    -1     
- Misses        422     423    +1     

[SkafteNicki]

Hi @kpoeppel,
I cannot reproduce the behaviour you are reporting on master

[kpoeppel]

@SkafteNicki :
I am sorry, somehow I added a wrong bug report code, it is updated now and I also checked it on master now.

@Borda:
Test is added now. For the specificity there is no error currently, but it is bad structure to assume that tn can be mutated.