title: How to find all argmax in ndarray
body: <p>I have a 2 dimensional NumPy ndarray.</p>
  
  <pre><code>array([[  0.,  20.,  -2.],
     [  2.,   1.,   0.],
     [  4.,   3.,  20.]])
  </code></pre>
  
  <p>How can I get all indices of the maximum elements? So I would like as output array([0,1],[2,2]).</p>