Merge pull request #19477 from LupusSanctus:am/eltwice_vec

* Aligned OpenCV DNN and TF sum op behaviour

Support Mat (shape: [1, m, k, n] ) + Vec (shape: [1, 1, 1, n]) operation
by vec to mat expansion

* Added code corrections: backend, minor refactoring

modules/dnn/include/opencv2/dnn/shape_utils.hpp

@@ -235,6 +235,23 @@ Range normalize_axis_range(const Range& r, int axisSize)
     return clamped;
 }
 
+static inline
+bool isAllOnes(const MatShape &inputShape, int startPos, int endPos)
+{
+    CV_Assert(!inputShape.empty());
+
+    CV_CheckGE((int) inputShape.size(), startPos, "");
+    CV_CheckGE(startPos, 0, "");
+    CV_CheckLE(startPos, endPos, "");
+    CV_CheckLE((size_t)endPos, inputShape.size(), "");
+
+    for (size_t i = startPos; i < endPos; i++)
+    {
+        if (inputShape[i] != 1)
+            return false;
+    }
+    return true;
+}
 CV__DNN_EXPERIMENTAL_NS_END
 }
 }