code: def test_linucb_training_step(self):
      self.trainer.training_step(self.batch, 0)
      self.trainer.on_train_epoch_end()
  
imports: import copy
  import unittest
  import numpy
  import numpy.testing
  import torch
  from libfb.py.collections import AttrDict
  from pytorch_lightning.loggers import TensorBoardLogger
  from reagent.core.types import CBInput
  from reagent.gym.policies.policy import Policy
  from reagent.gym.policies.samplers.discrete_sampler import GreedyActionSampler
  from reagent.models.linear_regression import LinearRegressionUCB
  from reagent.training.cb.linucb_trainer import LinUCBTrainer
  from reagent.training.cb.utils import add_chosen_arm_features
  from reagent.training.parameters import LinUCBTrainerParameters
  from torchrec.fb.metrics.meta_config import DefaultTaskInfo, MetricsConfig, QPSDef, RecMetricDef, RecMetricEnum
  from torchrec.fb.metrics.meta_metrics import generate_metric_module, RecMetricModule
label_desc: Use image node_id 2 for calling the TestLinUCB obj's underlying member method code with example usage: obj.test_linucb_training_step() without return types