message: Better assertion message in vectorizing_map.
  One situation that triggers this assertion is when trying to tf.print(pfor_config.collect(something)).  Arguably that should just work, but failing that, having the error indicate the nature of the problem is helpful.
diff: @@ -993,7 +993,8 @@ class PForConfig(object):
  def _lookup_reduction(self, pl):
  """Lookups Placeholder `pl` in the reduction map."""
  - assert isinstance(pl, ops.Tensor)
  + msg = "Expected Tensor, got {} of type {}."
  + assert isinstance(pl, ops.Tensor), msg.format(pl, type(pl))
  return self._reduce_concat_map.get(pl, None)
  