title: tensorflow reduce_mean with multidimension second argument
body: <p>I met the usage of the reduce_mean with the vector as the second arguments. I looked through sensor flow manual but can't find the corresponding example. The codes are below:</p>
  
  <pre><code>tf.reduce_mean(train, [0,1,2]
  </code></pre>
  
  <p>where train is at size batchsize x H x L x 2
  I also played with some experiments but can't figure out how this second vector input will be processed</p>
  
  <pre><code>tensor = tf.constant([[[2,2,4],[2,2,0]],[[2,2,0],[2,2,0]]])
  trainenergy = tf.reduce_mean(tensor, [0,1,2])
  Output = 1
  tensor = tf.constant([[[2,2,4],[2,2,0]],[[2,2,0],[2,2,0]]])
  trainenergy = tf.reduce_mean(tensor, [0])
  Output = [[2 2 2]
   [2 2 0]]
  tensor = tf.constant([[[2,2,4],[2,2,0]],[[2,2,0],[2,2,0]]])
  trainenergy = tf.reduce_mean(tensor, [0,1])
  Output = [2 2 1]
  </code></pre>