better trl parser with yaml config

[mnoukhov]

• correctly overrides yaml config with command line arguments do this by changing the defaults to values in the yaml so we get dataclass default < yaml config < command line arguments

• adds return_remaining_strings

• when return_remaining_strings is False, raises error if yaml contains extra args that are not in the dataclasses

• removes the need to have a config field in one of your dataclasses (I can revert this if this is preferred)

• simpler and cleaner than previous yaml parsing without the need for merging dataclasses

Fixes #1733

@younesbelkada let me know if you have comments or suggestions!

[HuggingFaceDocBuilderDev]

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