title: Tensorflow: best way to vertically stack matrices?
body: <p>Let say I have a 3D tensor representing several 2D matrices:</p>
  
  <pre><code>3D_tensor = tf.random_uniform([N0, N1, N2], 1, -1))
  </code></pre>
  
  <p>I want to stack vertically these 2D matrices such that the resulting 2D tensor would be of dimensions [N0+N1, N2]. WHat is the best way to do that ? </p>
  
  <p>The numpy process would be the following one:</p>
  
  <pre><code>3D = np.array([ [[1,2],[3,4]] , [[1,2],[3,4]] ]) # 3D.shape = (2,2,2)
  2D = np.vstack(3D) # D2.shape = (4,2)
  </code></pre>