title: Graph usage pattern for RNNs
body: <p>I am confused about the lifetime of tensors/ops/variables inside the default graph. </p>
  
  <p>In RNNs, the graph structure will change on each iteration due to the variable length of input data. Should separate graphs thus be created/destroyed on each iteration of training to accommodate the changing structure of the graph, or is there a way to stick with the default graph?</p>
  
  <p>Also, if new graphs are constructed, how can they share variables?</p>