[MRG+1] enable to use get_n_splits of LeaveOneGroupOut and LeavePGroupsOut...

[MRG+1] enable to use get_n_splits of LeaveOneGroupOut and LeavePGroupsOut with dummy parameters (#8794)

* remove needless argument checking

* add parameter checking as in LeavePGroupsOut

* add examples with dummy inputs

* add unittest for a get_n_splits method in LeaveOneGroupOut and LeavePGroupsOut classes

* X and y can be ommited in a get_n_splits function.

* fix error messages

* update examples

* fix test for an error message

* Revert "fix test for an error message"

This reverts commit 68b984207c704de1a3411d9ded68a11eba1e56f3.

* fix test for an error message

* fix error messages

* remove tailing white spaces

* add periods to messages

* test for ValueError’s of get_n_splits methods of LeaveOneOut / LeavePOut classes

* fix documents:
* parameter name: group -> groups
* modfy white space