code: def __init__(self):
      self.num_classes = 2
  
imports: import os
  import sys
  import unittest
  import torch
  import numpy
  from torch_geometric.data import Data
  from torch_points3d.models.panoptic.structures import PanopticResults, PanopticLabels
  from torch_points3d.metrics.panoptic_tracker import _Instance, InstanceAPMeter, PanopticTracker
label_desc: Use image node_id 1 to create a new MockDataset object  with example: obj = MockDataset()