title: keras backend conv2d error
body: <p>I am trying to use the <code>keras.backend.conv2d</code> function with my own kernel and I am getting the following error.</p>
  
  <pre><code>img = data[0,:,0]
  img = img.reshape(240, 320)
  
  kernelX = np.array([[-1, 1], 
                      [-1, 1]])
  
  img = K.constant(img)
  kernelX = K.constant(kernelX)
  
  I_x = K.conv2d(img, kernelX)
  </code></pre>
  
  <blockquote>
    <p>ValueError: number of input channels does not match corresponding
    dimension of filter, 320 != 2</p>
  </blockquote>
  
  <p>Could somebody please push me in the right direction?</p>