title: Implement 2D convolution using FFT
body: <p><code>TensorFlow.conv2d()</code> is impractically slow for convolving large images with large kernels (filters). It takes a few minutes to convolve a 1024x1024 image with a kernel of the same size. For comparison, <code>cv2.filter2D()</code> returns the result immediately.</p>
  
  <p>I found <code>tf.fft2()</code> and <code>tf.rfft()</code>.</p>
  
  <p>However it was not clear to me how to perform simple image filtering with these functions.</p>
  
  <p>How can I implement fast 2D image filtering with TensorFlow using FFT?</p>