code: def cb_training_step(
      self, batch: CBInput, batch_idx: int, optimizer_idx: int = 0
  ):
      self.scorer.learn(batch)
  
file_path: ReAgent/reagent/training/cb/mab_trainer.py
imports: import logging
  from reagent.core.types import CBInput
  from reagent.gym.policies.policy import Policy
  from reagent.models.mab import MABBaseModel
  from reagent.training.cb.base_trainer import BaseCBTrainerWithEval
label_desc: Use image node_id 2 for calling the MABTrainer obj's underlying member method code with example usage: obj.cb_training_step(batch, batch_idx, optimizer_idx) without return types