code: def n_arms(self) -> int:
      return len(self.expected_rewards)
  
imports: from abc import ABC, abstractmethod
  from functools import partial
  from multiprocessing import Pool
  from typing import Dict, List, Optional, Tuple, Type, Union
  import numpy
  import torch
  from reagent.mab.mab_algorithm import MABAlgo
  from torch import Tensor
label_desc: Use image node_id 3 for calling the MAB obj's underlying member method code with example usage: obj.n_arms() and returns: len