Make positional arguments required in the argument parser

[fschlatt]

🚀 Feature

Make positional arguments required for classes passed into the add_argparse_args function.

Motivation

I use the add_argparse_args function not only for the Trainer but also for additional classes required for my training script (e.g. the DataModule). Some arguments of the additional classes are positional and do not have suitable default values. In the current implementation, positional arguments receive a default value of inspect._empty, which usually leads to unforeseen errors somewhere further down the line.

Pitch

In the ptl argument parser utilities, modify the parser such that positional arguments of a class given to the add_argparse_args are marked as required in the resulting ArgumentParser.

cc @Borda

[carmocca]

Thanks for the suggestion!

Let me also shamelessly plug the LightningCLI which already takes care of this 💜