CI: add multi-GPU

[Borda]

What does this PR do?

add multi-GPU CI
Fixes #553

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[pep8speaks]

Hello @Borda! Thanks for updating this PR.

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

Comment last updated at 2021-03-10 12:11:44 UTC

[codecov]

Codecov Report

Merging #585 (e14a75d) into master (41ebb3e) will decrease coverage by 2.53%.
The diff coverage is 55.26%.

@@            Coverage Diff             @@
##           master     #585      +/-   ##
==========================================
- Coverage   76.59%   74.05%   -2.54%     
==========================================
  Files         117      117              
  Lines        6852     6864      +12     
==========================================
- Hits         5248     5083     -165     
- Misses       1604     1781     +177     

Flag	Coverage Δ
cpu	26.30% <50.00%> (+0.10%)	⬆️
pytest	26.30% <50.00%> (+0.10%)	⬆️
unittests	73.54% <55.26%> (-2.54%)	⬇️

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Impacted Files	Coverage Δ
pl_bolts/datasets/imagenet_dataset.py	20.00% <ø> (-0.46%)	⬇️
...lts/models/self_supervised/simclr/simclr_module.py	57.84% <ø> (-13.73%)	⬇️
pl_bolts/models/self_supervised/ssl_finetuner.py	24.13% <ø> (ø)
pl_bolts/models/self_supervised/cpc/cpc_module.py	21.25% <11.76%> (-0.17%)	⬇️
..._bolts/models/self_supervised/cpc/cpc_finetuner.py	19.35% <50.00%> (ø)
..._bolts/models/self_supervised/moco/moco2_module.py	64.18% <50.00%> (-7.44%)	⬇️
pl_bolts/models/self_supervised/__init__.py	100.00% <100.00%> (ø)
...bolts/models/self_supervised/amdim/amdim_module.py	91.26% <100.00%> (-5.48%)	⬇️
...l_bolts/models/self_supervised/byol/byol_module.py	60.24% <100.00%> (-22.69%)	⬇️
pl_bolts/models/self_supervised/cpc/__init__.py	100.00% <100.00%> (ø)
... and 11 more

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