Fix: ensure user-provided labels are retained in self._signature_columns

What does this PR do?

Fixes:
The _set_signature_columns_if_needed in sft_trainer.py overrides that of trainer.py in transformer, but it mistakenly ignores labels which makes it unable to train when the user selects skip_prepare_dataset=True in SFTConfig. Previous version (e.g 0.14.0) does not have an override version of _set_signature_columns_if_needed in sft_trainer.py.

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