📑 Refactor TrlParser

[qgallouedec]

What does this PR do?

First step part of #2380

• Test TrlParser
• Deprecate YamlConfigParser (not used outside TrlParser, only get the piece of code and add it to TrlParser)
• Update TrlParser documentation
• deprecate post_process_dataclasses (no-op since we removed XXXScriptArgument. We will find another way of passing gradient_checkpointing_use_reentrant
• Remove config arg from ScriptConfig (not compatible with TrlParser)

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

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[lewtun]

Nice harmonisation with the parser @qgallouedec ! LGTM with an ask to include 1 more test