code: def save_checkpoint(self, checkpoint_dir: str):
      return None
  
file_path: ray/python/ray/tune/examples/tf_mnist_example.py
imports: import argparse
  import os
  from filelock import FileLock
  from tensorflow.keras.layers import Dense, Flatten, Conv2D
  from tensorflow.keras import Model
  from tensorflow.keras.datasets.mnist import load_data
  from ray import train, tune
label_desc: Use image node_id 2 for calling the MNISTTrainable obj's underlying member method code with example usage: obj.save_checkpoint(checkpoint_dir) and returns: None