title: Conversion of tensor shape values to None in a keras model
body: <h2>Suppose I have a simple keras model as follows,</h2>
  <pre><code>Model: &quot;sequential_1&quot;
  _________________________________________________________________
  Layer (type)                 Output Shape              Param #   
  =================================================================
  conv2d (Conv2D)              (None, 16, 16, 8)         224       
  _________________________________________________________________
  conv2d_1                     (None, 7, 7, 8)           584       
  _________________________________________________________________
  conv2d_2                     (None, 3, 3, 2)           584       
  =================================================================
  Total params: 1,392
  Trainable params: 1,392
  Non-trainable params: 0
  </code></pre>
  <h2>Now I want the final output layer shape as follows,</h2>
  <pre><code>Model: &quot;sequential_1&quot;
  _________________________________________________________________
  Layer (type)                 Output Shape              Param #   
  =================================================================
  conv2d (Conv2D)              (None, 16, 16, 8)         224       
  _________________________________________________________________
  conv2d_1                     (None, 7, 7, 8)           584       
  _________________________________________________________________
  conv2d_2                     (None, None, None, 2)     584       
  =================================================================
  Total params: 1,392
  Trainable params: 1,392
  Non-trainable params: 0
  </code></pre>
  <h2>Code</h2>
  <pre><code>m = Sequential()
  
  m.add(layers.InputLayer(input_shape=(32, 32, 1)))
  m.add(Conv2D(8, (3,3), padding='same', strides=2))
  m.add(Conv2D(8, (3,3), strides=2))
  m.add(Conv2D(2, (3,3), strides=2))
  </code></pre>
  <h2>Details</h2>
  <blockquote>
  <p>I hope you got an understanding of the issue.
  Please try to make your own versions of the code. This is just a demo code.</p>
  </blockquote>
  <p>I am working on image colorization problem where we input an image with one color channel and get output with 2 color channels.
  The original code is huge and different from any codes on the internet. So please don't be concerned about suggesting me any image colorization codes.</p>
  <p>I really appreciate it if you can help me with this issue.</p>