Refactor LightningDataParallel

[awaelchli]

What does this PR do?

Fixes #5506
Follow-up to #5185

It is a bit hard to see what happens in the diff, because there are a lot of deletions.
I recommend to view diff in split view: Go to Files changed tab -> settings wheel -> change unifided to split

The refactor for DP here is basically analogeous to the refactor I did in #5185, with the addition that in DP we need to 1) convert Python scalars to tensors (to match functionality we had before) and 2) automatically unsqueeze 0-dim tensors

Todo: write tests

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[codecov]

Codecov Report

Merging #5670 (6a22d1d) into release/1.2-dev (5d239cc) will decrease coverage by 0%.
The diff coverage is 97%.

@@               Coverage Diff               @@
##           release/1.2-dev   #5670   +/-   ##
===============================================
- Coverage               89%     89%   -0%     
===============================================
  Files                  169     173    +4     
  Lines                12415   12339   -76     
===============================================
- Hits                 11099   11015   -84     
- Misses                1316    1324    +8     

[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 2021-01-31 07:19:40 UTC

[ananthsub]

LGTM!

[awaelchli]

Thanks for the reviews.
@Borda Unfortunately, there is an existing DP test failing which I haven't been able to figure out yet, so the PR is not yet ready to merge. So I am removing the ready label for now

[awaelchli]

@Borda Tests passing now, and I fixed the cyclic import error. This should be good now :)