class_docstr: Example one person world that uses only user input.
class_docstr_tok: ["Example","one","person","world","that","uses","only","user","input","."]
code: def assign_roles(agents):
      agents[0].disp_id = "EchoAgent"
  
imports: from parlai.core.worlds import World
  from parlai.chat_service.services.messenger.worlds import OnboardWorld
label_desc: Use image node_id 3 for calling the MessengerEchoTaskWorld obj's underlying member method code with example usage: obj.assign_roles(agents) without return types