code: def __len__(self):
      return len(self.coco_dataset)
  
file_path: mmf/mmf/datasets/builders/coco/detection_dataset.py
imports: import os
  from typing import Dict
  import torch
  import torch.nn.functional
  import torchvision
  from mmf.common.sample import Sample
  from mmf.datasets.base_dataset import BaseDataset
  from mmf.utils.distributed import gather_tensor_along_batch, object_to_byte_tensor
  from torch import nn, Tensor
label_desc: Use image node_id 3 for calling the DetectionCOCODataset obj's underlying member method code with example usage: obj.__len__() and returns: len