code: def _get_reference(self):
          
          super()._get_reference()
          self.hole_body_id = self.sim.model.body_name2id("hole")
          self.cyl_body_id = self.sim.model.body_name2id("cylinder")
docs: Sets up references to important components. A reference is typically an
          index or a list of indices that point to the corresponding elements
          in a flattened array, which is how MuJoCo stores physical simulation data.