🐛 fix doc test error

   ...   dataset=dataset, frac_train=0.6, frac_valid=0.2, frac_test=0.2

   ... )

   >>> len(train_dataset)

   >>> 3

   3

   >>> len(valid_dataset)

   >>> 1

   1

   >>> len(test_dataset)

   >>> 1

   1

Here, we've used the :code:`RandomSplitter` and splitted the data randomly

in the ratio of train:valid:test = 3:1:1. But, the random splitting sometimes