fix to kahan summation in ref kernel (#2140)

src/ngraph/runtime/reference/sum.hpp

@@ -35,21 +35,24 @@ namespace ngraph
                      const AxisSet& reduction_axes)
             {
                 CoordinateTransform output_transform(out_shape);
+                std::vector<T> c(shape_size(out_shape));
 
                 for (const Coordinate& output_coord : output_transform)
                 {
                     out[output_transform.index(output_coord)] = 0;
+                    c[output_transform.index(output_coord)] = 0;
                 }
 
                 CoordinateTransform input_transform(in_shape);
 
-                T c = 0;
                 for (const Coordinate& input_coord : input_transform)
                 {
                     Coordinate output_coord = reduce(input_coord, reduction_axes);
-                    T y = arg[input_transform.index(input_coord)] - c;
+                    T y = arg[input_transform.index(input_coord)] -
+                          c[output_transform.index(output_coord)];
                     T t = out[output_transform.index(output_coord)] + y;
-                    c = (t - out[output_transform.index(output_coord)]) - y;
+                    c[output_transform.index(output_coord)] =
+                        (t - out[output_transform.index(output_coord)]) - y;
                     out[output_transform.index(output_coord)] = t;
                 }
             }