title: In Tensorflow's C++ TensorShape API, what is the equivalent of Python's None?
body: <p>Suppose I have a tensor created with the Python API of Tensorflow, as follows,</p>
  
  <pre><code>x = tf.placeholder("float", shape=[None, inputLen])
  </code></pre>
  
  <p>I would like to create a tensorflow::Tensor equivalent of that in C++, so that I can run a trained graph that takes x as the input. What should I do with the first dimension of the input shape, which in C++ is of type tensorflow::TensorShape? </p>
  
  <p>If I do: </p>
  
  <pre><code>tensorflow::TensorShape inputShape;
  inputShape.AddDim(0);
  inputShape.AddDim(inputLen);
  </code></pre>
  
  <p>it doesn't seem to work, because num_elements becomes 0, which is not the value of inputLen that I expect. </p>