📋 Allow calling trl cli in sft mode with config file

[CloseChoice]

What does this PR do?

This PR makes it possible to specify all required arguments in a config.yaml file and call this via cli, e.g.

trl sft --config examples/cli_configs/example_config_dummy.yaml --output_dir test-trl-cli --lr_scheduler_type cosine_with_restarts

This line is directly from the trl docs but didn't work since config is no known argument of the TrlParser when calling parse_args which is always called.

Fixes #3102

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[CloseChoice]

maybe we should check here that dataset_name is in args? I needed to make dataset_name optional so that a call to parser.parse_args() works in the cases further down, checking that args contains dataset_name will then make sure that the same checks are in place as without this PR.

[qgallouedec]

maybe we should check here that dataset_name is in args

if you don't provide the dataset, it would fail when trying to load it in the sft script. I think the error will be pretty self-explanatory, no need to add an extra check in my opinion.

[qgallouedec]

Thanks, your solution is very good, I simplified the test a bit, and the parsing logic, and once the tests are green, we can merge 🎉

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[CloseChoice]

@qgallouedec thanks for the review and improving the PR. Pipelines are passing ;)