class_docstr: Map a function over the elements in a dataset.
class_docstr_tok: ["Map","a","function","over","the","elements","in","a","dataset","."]
code: def __len__(self):
      return len(self._dataset)
  
imports: import contextlib
  import copy
  import itertools
  import logging
  import numpy
  import pickle
  import random
  from typing import Callable, Union
  import torch
  import torch.utils.data
  from torch.utils.data.sampler import Sampler
  from detectron2.utils.serialize import PicklableWrapper
label_desc: Use image node_id 4 for calling the MapDataset obj's underlying member method code with example usage: obj.__len__() and returns: len