title: Depth-wise concatenate in Keras
body: <p>I'm trying to depth-wise concat (example of implementation in <a href="https://github.com/yunjey/stargan/blob/master/model.py#L58" rel="nofollow noreferrer">StarGAN</a> using Pytorch) a one-hot vector into an image input, say</p>
  
  <pre><code>input_img = Input(shape = (row, col, chann))
  one_hot = Input(shape = (7, ))
  </code></pre>
  
  <p>I stumbled on the same problem before (<a href="https://stackoverflow.com/questions/55116732/keras-replicating-1d-tensor-to-create-3d-tensor">it was class indexes</a>), and so I used RepeatVector+Reshape then Concatenate. But I found RepeatVector is not compatible when you want to repeat 3D into 4D (included <code>batch_num</code>).</p>
  
  <p>How do I implement this method in Keras? I found that <code>Upsampling2D</code> could do the works, but I don't know if it able to keep the one-hot vector structure during upsampling process</p>