Enhance HfArgumentParser functionality and ease of use
Author: konstantinjdobler
Pull request: yes

# What does this PR do?

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I have enhanced the `HfArgumentParser` for my own work and decided to contribute my changes back to `transformers`.

The changes are:
- Allow aliases for command line flags to be created (by providing `aliases` inside the metadata dict)
- Enable specification of one or multiple config files via a customizable command line flag (e.g. `--cfg ./path/to/basic-config.txt --cfg ./path/to/special.txt`). For my own use, I set the customizable command line flag to `"--cfg"` by default. I omitted this here to keep 100% backwards compatibility but a sensible default might make sense as well.
- Enable use of the `Literal` type for easy ad-hoc specification of choices without the need to define an `Enum` (inspired by [typed-argument-parser](https://github.com/swansonk14/typed-argument-parser)):
```python
@dataclass
class Args:
    literal_arg: Literal["the meaning of life", 42, "hitchhiking"] = 42
```
- Allow use of mixed types for `Enum` similar to `Literal`
- Created `HfArg` helper for a more concise syntax when creating data class fields:
```python
@dataclass
class Args:
    field_arg: int = field(default=42, metadata={"aliases": ["--arg", "-a"], "help": "This is a bit verbose."})
    hf_arg: int = HfArg(default=42, aliases=["--arg", "-a"], help="This is more concise.")
```



## Before submitting
- [ ] 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?
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- [x] Did you write any new necessary tests?


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Comments:
_The documentation is not available anymore as the PR was closed or merged._