class_docstr: Dataset of a dictionary of datasets.
  
  It combines multiple datasets into one dataset. Each example is represented
  by a dictionary mapping a key to an example of the corresponding dataset.
  
  Args:
      datasets: Underlying datasets. The keys are used as the keys of each
          example. All datasets must have the same length.
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code: def __len__(self):
      return self._length
  
imports: import six
label_desc: Use image node_id 3 for calling the DictDataset obj's underlying member method code with example usage: obj.__len__() and returns: self