title: Interleaving multiple TensorFlow datasets together
body: <p>The current TensorFlow dataset interleave functionality is basically a interleaved flat-map taking as input a single dataset. Given the current API, what's the best way to interleave multiple datasets together? Say they have already been constructed and I have a list of them. I want to produce elements from them alternatively and I want to support lists with more than 2 datasets (i.e., stacked zips and interleaves would be pretty ugly).</p>
  
  <p>Thanks! :)</p>
  
  <p>@mrry might be able to help.</p>