title: Reshape image to [-1, 1] with ImageDataGenerator
body: <p>Is it possible to reshape images in <code>[0, 255]</code> to <code>[-1, 1]</code> using <code>ImageDataGenerator</code>?</p>
  
  <p>I have seen that I can multiply the image with a value using the reshape parameter, but that only gives me the possibility to reshape it to <code>[0, 1]</code></p>