title: How does `tf.image.decode_image` deal with channels in Tensorflow?
body: <p>Does the <code>channels</code> in <code>tf.image.decode_image</code> must equal to the real channel of images? What would happen if the real channel is greater or less than the <code>channels</code> in <code>tf.image.decode_image</code>?
  If they must be equal, how should I deal with the situation that my image set contains images with different channels?</p>