title: Conv4D in tf.nn.convolution
body: <p>I tried using tf.nn.convolution() for 6D input with 4D filter. 
  But tf.nn.convolution() say "ValueError: <code>input</code> and <code>filter</code> must have rank at least 3 and at most 5".</p>
  
  <p>My question is, Where is this limitation from?
  Only the implementation of Tensorflow? or limitation of CUDA/cuDNN?</p>
  
  <p>(python 3.6, tensorflow 1.12)</p>