code: def register(cls, panel):
          
          panel_class = Horizon.register_panel(cls, panel)
          # Support template loading from panel template directories.
          panel_mod = import_module(panel.__module__)
          panel_dir = os.path.dirname(panel_mod.__file__)
          template_dir = os.path.join(panel_dir, "templates")
          if os.path.exists(template_dir):
              key = os.path.join(cls.slug, panel.slug)
              loaders.panel_template_dirs[key] = template_dir
          return panel_class
docs: Registers a :class:`~horizon.Panel` with this dashboard.