title: How to create Keras ZeroTensor of specific shape
body: <p>I am a total beginner with tensorflow.keras and I am wondering how I could create a constant zero tensor of a specific shape.</p>
  
  <p>For example with this:</p>
  
  <pre><code>zeros = tf.keras.backend.zeros((someTensor.shape[0], someTensor.shape[1], someTensor.shape[2], channels))
  concat = tf.kerasbackend.concatenate([someTensor, zeros], axis=3)
  </code></pre>
  
  <p>The operation tf.keras.backend.zeros fails with:</p>
  
  <pre><code>ValueError: Cannot convert a partially known TensorShape to a Tensor
  </code></pre>
  
  <p>I guess thats because the batch size is unknown during graph building. How can I create a ZeroTensor or any other constant tensor when I don't know the batchsize at that moment? Or is there some kind of unknown(?) value that I can specify?</p>