[metrics] Accuracy error fix

[SkafteNicki]

What does this PR do?

Fixes #3006 and #2305
Throwing an error in accuracy if target tensor only consist of zeros is counter intuitive to many user and is also different from how sklearn handles this case. This PR remove this constrain.

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[pep8speaks]

Hello @SkafteNicki! Thanks for updating this PR.

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2020-10-01 08:47:34 UTC

[mergify]

This pull request is now in conflict... :(

[codecov]

Codecov Report

Merging #3764 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #3764   +/-   ##
======================================
  Coverage      89%     89%           
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  Files         110     110           
  Lines        8554    8554           
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  Hits         7637    7637           
  Misses        917     917