get_partial_sum Function

  function get_partial_sum(this, upstream_grad) result(output)
    implicit none
    class(array_type), intent(inout) :: this
    type(array_type), intent(in) :: upstream_grad
    type(array_type) :: output
    type(array_type), pointer :: ptr

    if(this%is_forward)then
       ptr => sum( &
            upstream_grad, &
            dim = this%indices(1) &
       )
    else
       ptr => spread( &
            upstream_grad, &
            dim=this%indices(1), &
            index=this%indices(2), &
            ncopies= size(this%left_operand%val, this%indices(1)) &
       )
    end if
    call output%assign_and_deallocate_source(ptr)
  end function get_partial_sum