I was searching the same stuff. To me, it seems the y should be something like:
for a task, each sample can have at most 3 labels:
[0,0,1] ---> y = [2,-1,-1]
[1,0,1] ---> y = [0, 2,-1]
[0,1,1] ---> y = [1, 2,-1]

y includes the index of the target labels, and padded with negative values if num of labels is less than 3(in this example).
Note: I haven’t tested this.