[DPO] DPOConfig class

[kashif]

Add a DPOConfig training argument dataclass together with deprecation warnings to the DPOTrainer

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[lewtun]

Thanks for the refactor and new config @kashif ! Overall LGTM with some nits about moving all args from the trainer init to the config in the script example

As with the SFT trainer, it would be good to have a small integration test that checks we have the same trainer if it is initialised in both ways (via args and via init) to ensure backwards compat

[younesbelkada]

Great work thanks @kashif ! LGTM once @lewtun comments are addressed !

[lewtun]

Thanks for iterating @kashif ! I left one turbo nit and then this can be merged after addressing it :)