title: fill lower triangle (including diagonal) of a Numpy Array with -infinity - Python
body: <p>I have a 2D array as below.</p>
  <pre><code>[[1 2 3]
   [4 5 6]
   [7 8 9]]
  </code></pre>
  <p>I need it to be converted into:</p>
  <pre><code>[[-inf  2   3] 
   [-inf -inf 6]
   [-inf -inf -inf]]
  </code></pre>
  <p>i.e., fill the lower triangle, including diagonal, to -infinity.
  How to do it using python ? Please help.</p>