title: sum numpy ndarray with 3d array along a given axis 1
body: <p>I have an numpy ndarray with shape (2,3,3),for example:</p>
  
  <pre><code>array([[[ 1,  2,  3],
      [ 4,  5,  6],
      [12, 34, 90]],
  
     [[ 4,  5,  6],
      [ 2,  5,  6],
      [ 7,  3,  4]]])
  </code></pre>
  
  <p>I am getting lost in np.sum(above ndarray ,axis=1), why that answer is:</p>
  
  <pre><code>array([[17, 41, 99],
     [13, 13, 16]])
  </code></pre>
  
  <p>Thanks</p>