message: Generalize xla.is_flat to include any Mapping type, not just dict.
  This allows custom mapping types to be returned from functions used with xla.compile.
diff: @@ -429,11 +429,12 @@ def is_flat(outputs):
  # there is, then outputs is non-flat.
  if isinstance(outputs, collections.Sequence):
  for o in outputs:
  - if isinstance(o, collections.Sequence) or isinstance(o, dict):
  + if isinstance(o, collections.Sequence) or isinstance(
  + o, collections.Mapping):
  return False
  # If outputs is a dict, it is non-flat.
  - if isinstance(outputs, dict):
  + if isinstance(outputs, collections.Mapping):
  return False
  # Getting here means either outputs itself is a single non-structured value
  