up then down reordering, given the operator has the default even-odd ordering.

Otherwise this function will reorder indices where all even indices now come before odd indices.

Example: 0,1,2,3,4,5 -> 0,2,4,1,3,5

The function takes in the index of the mode that will be relabeled and the total number modes.

mode_idx int

the mode index that is being reordered

num_modes int

the total number of modes of the operator.

Returns (int): reordered index of the mode.