title: tensorflow: modify the shape of placeholder
body: <p>I meet this problem during my coding. I have a placeholder </p>
  
  <pre><code>x = tf.placeholder(tf.float32,[None,128])
  </code></pre>
  
  <p>I want to generate some noise with dimension 10 and concatenate with x. The number of noises should match the number of xs that shows as None here and will be provided later. In numpy I would do it as:</p>
  
  <pre><code>noise_shape=[x.shape[0],10]
  noise = tf.random_normal(shape=noise_shape)
  out = tf.concat(axis=1, values=[x, noise])
  </code></pre>
  
  <p>But in tensorflow, [x.shape[0],10] doesn't work. Can anyone please light up me how to achieve this? </p>