abstract fun itemVisibility(groupCount: Int, groupIndex: Int): VisibilityManager.VisibilityType
Defines a visibility for each group of dividers.
groupCount - number of groups in a list (equal to the list size when the span count is 1).
groupIndex - position of the group (equal to the item position when the span count is 1).
Return
the type of visibility related to the current group of dividers.