title: 3d to 2d tensor using tf.gather_nd in tensorflow
body: <p>I want to extract elements from 3d-tensor to make 2d-tensor.
  I have 3d-tensor (2000 X 2 X 3) and indices 1d-tensor for 3-dim (2000).
  The indices tensor surely contain indices 0~2 of 2000 elements.</p>
  
  <p>In fact, I want same result with <code>A[:,:,inds]</code>.
  How to do this with <code>tf.gather_nd</code>, please, help me.</p>