class_docstr: A agent without train/eval_step.
  
  Useful if you only want preprocessing.
code: def eval_step(self, batch):
      return Output()
  
file_path: ParlAI/parlai/agents/test_agents/null.py
imports: import torch
  from parlai.core.torch_agent import TorchAgent, Output
label_desc: Use image node_id 5 for calling the NullAgent obj's underlying member method code with example usage: obj.eval_step(batch) and returns: Output