class_docstr: A simple adapter
  converting NiftyNet sampler's output into PyTorch Dataset properties
code: def __len__(self):
      return len(self.sampler.reader.output_list)
  
imports: import numpy
  import torch
  from torch.utils.data import Dataset
label_desc: Use image node_id 3 for calling the DatasetNiftySampler obj's underlying member method code with example usage: obj.__len__() and returns: len