title: How to use the dynamic shape of a tensor in tensorflow
body: <p>How can I build a tensorflow graph for the following computation? The question I'm having now is how to use the shape information of tensor A, which has a variable shape size. </p>
  
  <pre><code>A = tf.placeholder(tf.float32, [None,10])
  B = tf.Variable(tf.random_normal([10,20]))
  C = tf.matmul(A, B)
  D = tf.matmul(tf.transpose(C), A) # the value of A.shape[0]
  </code></pre>