code: def __init__(self):
      self.num_classes = 5
      self.class_to_segments = {"class1": [0, 1], "class2": [2, 3]}
      self.is_hierarchical = True
  
file_path: torch-points3d/test/test_shapenetparttracker.py
imports: import unittest
  import numpy
  import os
  import sys
  from torch_points3d.metrics.shapenet_part_tracker import ShapenetPartTracker
label_desc: Use image node_id 1 to create a new MockDataset object  with example: obj = MockDataset()