code: def loss(self):
      assert (
          self.mode != "infer"
      ), "invalid loss calculationg in infer mode"
      cost = fluid.layers.cross_entropy(
          input=self.network_outputs[0],
          label=self.label_input,
          ignore_index=-1,
      )
      self.loss_ = fluid.layers.mean(x=cost)
      return self.loss_
  
imports: import numpy
  import paddle.fluid
  from paddle.fluid import ParamAttr
  from ..model import ModelBase
  from .tsn_res_model import TSN_ResNet
  import logging
label_desc: Use image node_id 7 for calling the TSN obj's underlying member method code with example usage: obj.loss() and returns: self