Solved by tom in post #2 
              
              
                The gradient operator ∇ is in itself linear as an operator. This means that ∇a - ∇b = ∇(a-b). 
In fact, if you write a minus function m(x1, x2) = x1 - x2 and some loss l(m) calculated from that, then the backpropagation step for this calculation is just feeding dl/dm as grad_out into the backward o…