title: How to convert array from dtype=object to dtype=np.int
body: <p>Currently, the array I got is</p>
  
  <pre><code>arr = array([array([ 2,  7,  8, 12, 14]), array([ 3,  4,  5,  6,  9, 10]),
     array([0, 1]), array([11, 13])], dtype=object)
  </code></pre>
  
  <p>How can I convert it into <code>array([[ 2,  7,  8, 12, 14], [ 3,  4,  5,  6,  9, 10], [0, 1], [11, 13]])</code>?</p>
  
  <p>I tried <code>arr.astype(np.int)</code>, but failed</p>