HfArgumentParser: allow for hyhenated field names in long-options
Author: djmarti
Pull request: yes

argparse converts hyphens into underscores before assignment (e.g., an option passed as `--long-option` will be stored under `long_option`), so there is no need to pass options as literal attributes, as in `--long_option` (with an underscore instead of a hyphen). This commit ensures that the original `argparse`'s  behavior is respected by `parse_args_into_dataclasses` as well.

Fixes [Issue 33933](https://github.com/huggingface/transformers/issues/33933).

# What does this PR do?

Given a dataclass
```python
from dataclasses import dataclass

@dataclass
class Config:
    item_type: str
    item_length: int
```
`HfArgumentParser.parse_args_into_dataclasses` should correctly parse parameters like `--item-type` or `--item-length` (with words separated by *hyphens* instead of *underscores*). Previous versions were not able to parse them:
```python
from transformers import HfArgumentParser
parser = HfArgumentParser([Config])
parser.parse_args_into_dataclasses(args=["--item-type", "my_type", "--item-length", "23"])
```
would throw an error because the parser was expecting `--item_type` and `--item_length`. That's not ideal.

### Motivation

1. Consistency with `argparse`. [`argparse` converts hyphens into underscores before assignment](https://docs.python.org/dev/library/argparse.html#dest):
   >  If no long option strings were supplied, dest will be derived from the first short option string by stripping the initial - character. Any internal - characters will be converted to _ characters to make sure the string is a valid attribute name. 
2. It is standard convention to use hyphens in long-option parameters (e.g., `--file-type`, not `--file_type`).


- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?


Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_33990). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

Thanks for reviewing the code, @ArthurZucker. I am not sure what the next step is for me. The `ci/circleci: tests_hub` check is failing due to some errors unrelated to this PR:
```
HTTPError: 401 Client Error: Unauthorized for url: https://hub-ci.huggingface.co/Tomlim/myt5-base/resolve/main/tokenizer_config.json
```
Should I rebase from main and repush?

All good I was waiting for the other CIs to run! 


Thanks @djmarti 🤗