[qol] example scripts: parse args from .args file or JSON
You can either:
- pass the path to a json file as the unique argument 
- automatically load args from a .args file that's a sibling of the entrypoint script.

```json
{
  "model_name_or_path": "bert-base-cased",
  "task_name": "mnli",
  "data_dir": "./data/glue/MNLI",
  "output_dir": "./models/mnli"
}
```

```
--model_name_or_path distilroberta-base
--task_name mnli
```

I use the .args method myself, but I think both are ok.

Tagging @jplu and @stefan-it who have expressed interest in this.
Thanks for adding this! Could you also integrate it into the `run_ner` example 🤔<|||||>Yes @stefan-it I'll add it to all (Trainer-updated) scripts.

Do you have a preference between the JSON approach and the args one? Or should we do both?<|||||>Perfect!!! it works like a charm. I think both would be nice.<|||||>Both would be nice, thanks!<|||||>Very nice!<|||||>@julien-c Would it be helpful when I open a PR that adds this to the `run_ner` example, or do you already have this feature on a branch 🤔<|||||>I do not have a branch that does that currently so feel free to do it (+ also run_language_modeling and run_multiple_choice if you're so inclined :)<|||||>@julien-c I am a bit late to the party, but I went over it quickly nevertheless. As my comments show, if the only function of `trim_suffix` is to remove the extension or replace it, it might be more readable to simply use the built-in `Path.with_extension` functionality.

If you agree I can do a PR for this.

Apart from that: great stuff. especially with a growing number of parameters and options, a config file is really welcome!<|||||>@BramVanroy Yes, I didn't know this feature, PR is welcome.