title: Does TensorFlow allow definition of variables with undefined size?
body: <p>Does TensorFlow allow definition of variables with undefined size? For example I can easily define a placeholder with <code>None</code> values in shape - this means that these values can be specified at run time, after the graph is built. <em>But shapes of variables must be supplied before construction of the graph.</em> Is the previous statement true? If it is, why is it so?</p>