Return a list of optgroups for this widget.

def get_only_selected_choices(self, value):
        """Return a list of optgroups for this widget."""
        schoices = self.choices
        selected_choices = set([force_text(v) for v in value if v])
        if isinstance(schoices, ModelChoiceIterator):
            schoices.queryset = schoices.queryset.filter(pk__in=selected_choices)
        else:
            schoices = [e for e in schoices if force_text(e) in selected_choices]
        return schoices