[SFT] add SFT Trainer Config dataclass

[kashif]

Add a SFTConfig training argument dataclass together with deprecation warnings to the SFTTrainer

[HuggingFaceDocBuilderDev]

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[lewtun]

Thanks a lot for adding this config @kashif ! LGTM apart from some nits.

Given this is the most used trainer in the lib, would you mind adding a small backwards compatibility integration test where we init the trainer in two ways (setting all args we deprecated in the init) and then checking the results after 1 train step are the same?

[younesbelkada]

Thank you so much ! In addition to @lewtun comments i have two tiny comments, WDYT?

[younesbelkada]

Looking great thanks ! failing test is unrelated to this PR: https://github.com/huggingface/trl/actions/runs/8786654281/job/24110021581?pr=1530

[lewtun]

Looks great, with a minor nit on the dataset split names. Feel free to merge after fixing that!