title: In TensorFlow, how can I get nonzero values and their indices from a tensor with python?
body: <p>I want to do something like this.<br>
  Let's say we have a tensor A.  </p>
  
  <pre><code>A = [[1,0],[0,4]]
  </code></pre>
  
  <p>And I want to get nonzero values and their indices from it.  </p>
  
  <pre><code>Nonzero values: [1,4]  
  Nonzero indices: [[0,0],[1,1]]
  </code></pre>
  
  <p>There are similar operations in Numpy.<br>
  <code>np.flatnonzero(A)</code> return indices that are non-zero in the flattened A.<br>
  <code>x.ravel()[np.flatnonzero(x)]</code> extract elements according to non-zero indices.<br>
  Here's <a href="http://docs.scipy.org/doc/numpy-1.10.1/reference/generated/numpy.flatnonzero.html" rel="noreferrer">a link</a> for these operations.</p>
  
  <p>How can I do somthing like above Numpy operations in Tensorflow with python?<br>
  (Whether a matrix is flattened or not doesn't really matter.)</p>