[feature] leave-k-out split mode

[mkhe93]

• Introduced a new leave-k-out split mode, as already available in Elliot (see: Elliot paper).
• the leave-k-out mode allows for excluding a specified number of elements (denoted as k) from the dataset, based on the chosen mode
• updated the documentation to reflect this new split mode and provide usage instructions

Example in eval_args in example.yaml for k=10:

split: {'LK': ['test_only', 10]}

[mkhe93]

Created this mode during my researches and Recbole improved my workflow a lot.
I'm happy for feedback and improvements!