class_docstr: TowheeDataSet is a kind of dataset wrapper, where the `self.dataset` is the true dataset.
code: def __len__(self):
      raise NotImplementedError
  
file_path: towhee/towhee/data/dataset/dataset.py
imports: from torch.utils.data.dataset import Dataset
label_desc: Use image node_id 2 for calling the TowheeDataSet obj's underlying member method code with example usage: obj.__len__() without return types