code: def set_default_backend(default_dir, backend_name):
      os.makedirs(default_dir, exist_ok=True)
      config_path = os.path.join(default_dir, "config.json")
      with open(config_path, "w") as config_file:
          json.dump({"backend": backend_name.lower()}, config_file)
      print(
          'Setting the default backend to "{}". You can change it in the '
          "~/.dgl/config.json file or export the DGLBACKEND environment variable.  "
          "Valid options are: pytorch, mxnet, tensorflow (all lowercase)".format(
              backend_name
          )
      )
  
label_desc: Use image node_id 1 for calling a global function with example usage: set_default_backend(default_dir, backend_name) without return types