internal library initializer.

def _init_rabit():
    """internal library initializer."""
    if _LIB is not None:
        _LIB.RabitGetRank.restype = ctypes.c_int
        _LIB.RabitGetWorldSize.restype = ctypes.c_int
        _LIB.RabitIsDistributed.restype = ctypes.c_int
        _LIB.RabitVersionNumber.restype = ctypes.c_int