How to calculate the outer sum (not product) between 2 (1D) vectors

It seems like np.einsum should do the trick, but I haven’t been able to make it work.

An example:

a = np.arange(3)
b = np.arange(2)

#that computes the outer product
res = np.einsum('i,j->ij',a,b)

#The resulting array I am looking for is:
out = [[0, 1], [1, 2], [2, 3]]
#or its transpose. 

I’ve been searching and all functions seem to point towards outer product, not outer sum. A for loop would do the job, but I’d like to have something a lot more efficient than that.

Does anyone know how to implement the outer sum using np.einsum or something else?


Outer sum:

res = np.add.outer(a, b)