[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!

[TayTroye]

@mkhe93
Hello, thank you for your attention. The commonly used partitioning method in research study is leave-one-out, and you are welcome to use the leave-k-out method as needed. At this time, we do not have plans to integrate it into RecBole.