Method _flatten

Flattened attribute values in sorted order by attribute name. Modules are flattened by first walking their attributes in name order. Each attribute value is then flattened to find leaf values. If flatten is to be applied `recursive`ly then if the leaf is a `Module` it will also be f

(self,
               recursive=True,
               predicate=None,
               attribute_traversal_key=None,
               with_path=False)