computeOutputShape(inputShape) {
    return [inputShape[0], inputShape[1], inputShape[2], 2 * inputShape[3]]
  }
This layer only works on 4D Tensors [batch, height, width, channels],
and produces output with twice as many channels.

layer.computeOutputShapes must be overridden in the case that the output
shape is not the same as the input shape.
@param {*} inputShapes