running sum $\sum +$

Computes the running sum of the input tensor along a given axis. For example, take this rank 2 tensor:

$$\left( \begin{array}{cccc} 1& 2& 3& 4 \\ 5& 4& 3& 2 \\ 8& 7& 6& 5 \end{array} \right)$$

The running sum of this tensor, along the horizontal dimension, is:

$$\left( \begin{array}{cccc} 1& 3& 6& 10 \\ 5& 9& 12& 14 \\ 8& 15& 21& 26 \end{array} \right)$$