Add better support for HuggingFace-style dataclasses

[lebrice]

Fixes #169

TODOs left:

• [x] Test that the "help" entry gets used as the "--help" message
• [x] Test for that weird enum issue with the SchedulerType enum.
  • Unable to reproduce it, for some reason!
• [x] Test that this work with the TrainingArguments class from Huggingface. Potentially make an extra CI test job with it.
• [x] Add support for the "choices" in the metadata. Not sure if that's what they do
• [x] Fix the entire docstring of TrainingArguments being used in the help text. Fields should get those instead (related to #166 )

Signed-off-by: Fabrice Normandin fabrice.normandin@gmail.com