class_docstr: A agent without train/eval_step.
  
  Useful if you only want preprocessing.
code: def __init__(self, opt, shared=None):
      self.model = self.build_model()
      self.criterion = self.build_criterion()
      super().__init__(opt, shared)
  
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 1 to create a new NullAgent object from inherited base classes: TorchAgent with example: obj = NullAgent(opt, shared)