code: def __len__(self):
      # number of entries == number of unique image_ids in csv.
      return len(self._csv["image_id"].unique().tolist())
  
imports: import os
  import pandas
  import random
  import torch
  import torchvision
  from detecto.config import config
  from detecto.utils import default_transforms, filter_top_predictions, xml_to_csv, _is_iterable, read_image
  from torchvision import transforms
  from torchvision.models.detection.faster_rcnn import FastRCNNPredictor
  from tqdm import tqdm
label_desc: Use image node_id 2 for calling the Dataset obj's underlying member method code with example usage: obj.__len__() and returns: len