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 __getitem__(self, idx):
      super(ConcatDataset, self).__getitem__(idx)
      dataset_index = self.cum_sizes.searchsorted(idx, "right")
  
      if dataset_index == 0:
          dataset_idx = idx
      else:
          dataset_idx = idx - self.cum_sizes[dataset_index - 1]
  
      return self.datasets[dataset_index][dataset_idx]
  
imports: from .dataset import Dataset
  import numpy
label_desc: Use image node_id 3 for calling the ConcatDataset obj's underlying member method code with example usage: obj.__getitem__(idx) and returns: self