Fix argparse default value bug

[EspenHa]

What does this PR do?

Fixes #1916.

Essentially this PR will make parse_argparser properly do the inverse of what add_argparse_args does.

The reason for the issue is that PL auto-generates an argparser that accepts flag style options, i.e. --auto_lr_find leads to args.auto_lr_find is True. The only way to provide a negative flag (for options that default to True) is to use --auto_lr_find False. Because some parameters (like auto_lr_find) can be both a boolean and a string, the generated parser for these arguments uses nargs="?", which means that the default value becomes None, regardless of the default value in __init__. This means that for the flag-style options, parse_argparser needs to convert some of the None values to booleans.

Not sure if I should format with black or not, as that would create a large diff that is unrelated to the changes needed for the issue.

Also I think the argparse solution is not very robust. This PR simply fixes the immediate issue, but I think it is quite likely that something similar could happen again.

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PR review

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[codecov]

Codecov Report

Merging #2526 into master will increase coverage by 1%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #2526   +/-   ##
======================================
+ Coverage      88%     89%   +1%     
======================================
  Files          69      69           
  Lines        5628    5635    +7     
======================================
+ Hits         4963    5023   +60     
+ Misses        665     612   -53     

[Borda]

it took me awhile to understand it... :]

[EspenHa]

Yeah, it gets a bit complicated. Took me a while to understand it as well :P