argparse (used by hf_argparser) explicitly passes all defaults

[angus-lherrou]

Python 3.8, argparse.py:1793:

# add any parser defaults that aren't present
for dest in self._defaults:
    if not hasattr(namespace, dest):
        setattr(namespace, dest, self._defaults[dest])

This causes all parameters to be passed explicitly, including defaults.

Want to account for this for the transformers.hf_argparser use case