fix boolean argparse

[fschlatt]

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What does this PR do?

Fixes #1570 .

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[codecov]

Codecov Report

Merging #1571 into master will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1571   +/-   ##
======================================
  Coverage      89%     89%           
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  Files          68      68           
  Lines        3906    3906           
======================================
+ Hits         3471    3473    +2     
+ Misses        435     433    -2     

[williamFalcon]

@fschlatt awesome PR!
There's another issue where --gpus 1 or --gpus 8 does not work with this argparse method. want to look into that as well?

[fschlatt]

@williamFalcon @Borda I'd love fix the --gpus bug but unfortunately I'm not able to run the tests. I'm trying to execute them from WSL with the bash script. But I get the error No module named py from the line python -m coverage run --source pytorch_lightning -m py.test pytorch_lightning tests pl_examples -v --doctest-modules --flake8.