Fix result gathering with varying tensor shapes

[awaelchli]

What does this PR do?

Fixes #3019

The critical test fails on master, but I added also more tests for the old behaviour.

Solution:

• By default, we need to concatenate, because stacking would only work when the dataloader returns equal batch sizes. This is not the case in general.
• If the tensors are scalar, we stack.
• If stacking and concatenating both don't work, we gather into a list. At this point the user should be aware what they return and need to deal with it in their *_epoch_end.

Before submitting

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• Did you write any new necessary tests?
• Did you verify new and existing tests pass locally with your changes?
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PR review

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Did you have fun?

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[pep8speaks]

Hello @awaelchli! Thanks for updating this PR.

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

Comment last updated at 2020-08-18 23:56:42 UTC

[codecov]

Codecov Report

Merging #3020 into master will decrease coverage by 3%.
The diff coverage is 83%.

@@           Coverage Diff           @@
##           master   #3020    +/-   ##
=======================================
- Coverage      90%     87%    -3%     
=======================================
  Files          81      81            
  Lines        7644    8004   +360     
=======================================
+ Hits         6878    6967    +89     
- Misses        766    1037   +271     

[awaelchli]

Let me know if you think there is a better place to put these tests.

[Borda]

Let me know if you think there is a better place to put these tests.

the location for tests is fine :]