Add optional grouped parsers description to HfArgumentParser
Author: peteriz
Pull request: yes

# What does this PR do?

This PR adds optional grouping to the argument parser of `HfArgumentParser` when multiple dataclasses are used (with different sub-grouped parameters, such as optimizer setup, model config, etc.) so that the displayed `-h` will print multiple grouped arguments in a more semantically organized way.
Uses optional attribute `_argument_group_name=<some string>` in the dataclass. If exists in the dataclass a sub parser will be used instead of the root `HfArgumentParser`.

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## 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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@peteriz: Updated docstring inline

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@sgugger 

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Comments:
Thanks!