tensor(indices=tensor([[0, 2, 3]]),
             values=tensor([[-1.4512,  0.4073],
                            [-0.8901,  0.2017],
                            [-0.3183, -1.7539]]),
             size=(5, 2), nnz=3, layout=torch.sparse_coo)
  # when sum over all sparse dim, return a dense tensor
  # with summed dims squeezed
  >>> torch.sparse.sum(S, [0, 1, 3])
  tensor([-2.6596, -1.1450])
