code: def input_key(self):
      return self._input_key
  
imports: import logging
  from typing import Any, Dict, Sequence
  import torch
  import torch.nn
  from classy_vision.generic.util import is_pos_int, is_pos_int_list
  from .None import register_model
  from .classy_model import ClassyModel
  from .resnext3d_stage import ResStage
  from .resnext3d_stem import R2Plus1DStem, ResNeXt3DStem
label_desc: Use image node_id 7 for calling the ResNeXt3DBase obj's underlying member method code with example usage: obj.input_key() and returns: self