Auto convert to contiguous format for all_gather

[SkafteNicki]

What does this PR do?

Fixes #4781
When syncing using gather_all, tensors are required to be in contiguous memory format. This PR converts tensors that are not and warns the user.

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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 2020-12-05 13:58:36 UTC

[codecov]

Codecov Report

Merging #4907 (30a883a) into master (7234970) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #4907   +/-   ##
======================================
  Coverage      93%     93%           
======================================
  Files         129     129           
  Lines        9359    9359           
======================================
  Hits         8677    8677           
  Misses        682     682           

[tchaton]

LGTM !

[Borda]

@SkafteNicki mind check the latest comments? 🐰