title: Delayed objects of unspecified length are not iterable error when using multiple outputs from function
body: <p>This code: </p>
  
  <pre><code>@dask.delayed
  def sum_nr(a,b):
      return a+b, a, b
  
  for i in range(10):
      sum_ab, a, b = sum_nr(i, i+1)
  </code></pre>
  
  <p>produces</p>
  
  <pre><code>TypeError: Delayed objects of unspecified length are not iterable
  </code></pre>
  
  <p>What is a different way of outputting multiple items from a function, in dask? </p>