Fix consistency in the output of scalar tensors

[SkafteNicki]

What does this PR do?

Fixes #611
Trying to fix consistency in the output of metrics. If a metric returns a scalar tensor, then sometimes it is

tensor([...])

and other times it is:

tensor(...)

This PR fixes this by doing a squeeze after compute if the output has numel=1.

Before submitting

• Was this discussed/approved 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

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[codecov]

Codecov Report

Merging #622 (f1789e0) into master (02ea8de) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #622   +/-   ##
=====================================
  Coverage      95%    95%           
=====================================
  Files         155    155           
  Lines        5508   5513    +5     
=====================================
+ Hits         5236   5244    +8     
+ Misses        272    269    -3