[Feat] Add TORCH_DISTRIBUTED_BACKEND env variable

[tchaton]

What does this PR do?

This PR expose torch backend as an env variable for the user.

 PL_TORCH_DISTRIBUTED_BACKEND=gloo python train.py ...

Fixes #5355

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[pep8speaks]

Hello @tchaton! Thanks for updating this PR.

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

Comment last updated at 2021-02-17 16:10:55 UTC

[codecov]

Codecov Report

Merging #5981 (cd8de8f) into master (5157ba5) will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #5981   +/-   ##
======================================
- Coverage      93%     93%   -0%     
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  Files         160     160           
  Lines       11340   11340           
======================================
- Hits        10550   10521   -29     
- Misses        790     819   +29