code: def get_root_nodes(self, project, depth=None):
          
          route_values = {}
          if project is not None:
              route_values['project'] = self._serialize.url('project', project, 'str')
          query_parameters = {}
          if depth is not None:
              query_parameters['$depth'] = self._serialize.query('depth', depth, 'int')
          response = self._send(http_method='GET',
                                location_id='a70579d1-f53a-48ee-a5be-7be8659023b9',
                                version='5.1-preview.2',
                                route_values=route_values,
                                query_parameters=query_parameters)
          return self._deserialize('[WorkItemClassificationNode]', self._unwrap_collection(response))
docs: GetRootNodes.
          [Preview API] Gets root classification nodes under the project.
          :param str project: Project ID or project name
          :param int depth: Depth of children to fetch.
          :rtype: [WorkItemClassificationNode]