class_docstr: Dataset to concatenate multiple datasets.
  
  Purpose: useful to assemble different existing datasets, possibly
  large-scale datasets as the concatenation operation is done in an
  on-the-fly manner.
  
  Args:
      datasets (iterable): List of datasets to be concatenated
code: def __len__(self):
      return self.cum_sizes[-1]
  
imports: from .dataset import Dataset
  import numpy
label_desc: Use image node_id 2 for calling the ConcatDataset obj's underlying member method code with example usage: obj.__len__() and returns: self