Override add_argparse_args in the FlashTrainer

[carmocca]

What does this PR do?

Hacky fix, but I want to avoid having to implement support for subclasses in Lightning. There is jsonargparse for that.

Fixes #342

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[codecov]

Codecov Report

Merging #343 (0366f44) into master (2113c11) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #343      +/-   ##
==========================================
+ Coverage   88.59%   88.61%   +0.01%     
==========================================
  Files          80       80              
  Lines        4061     4066       +5     
==========================================
+ Hits         3598     3603       +5     
  Misses        463      463              

Flag	Coverage Δ
unittests	88.61% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flash/core/trainer.py	86.76% <100.00%> (+1.05%)	⬆️

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[ethanwharris]

Haha, it is hacky but it works, LGTM 😃