Completions Only Loss is incompatible with use_liger_kernel set as true

Hi Team, discovered that completions only loss does not work when use_liger_kernel is set as true, since this line drops the completion_mask. Is this an expected behavior? 

https://github.com/huggingface/trl/blob/65245f6be8dea3d559a58788eca439f9c2d0f1e2/trl/trainer/sft_trainer.py#L670