Fixing typing issues DataSet -> DataType (#1486)

Author: abhinavarora

torchtext/experimental/datasets/sst2.py

@@ -1,7 +1,7 @@
 # Copyright (c) Facebook, Inc. and its affiliates.
 import os
 
-from torch.utils.data.dataset import IterableDataset
+from torch.utils.data import IterDataPipe
 from torchtext._internal.module_utils import is_module_available
 from torchtext.data.datasets_utils import (
     _add_docstring_header,
@@ -56,7 +56,7 @@ def SST2(root, split, validate_hash=True):
     return SST2Dataset(root, split, validate_hash=validate_hash)
 
 
-class SST2Dataset(IterableDataset):
+class SST2Dataset(IterDataPipe):
     """The SST2 dataset uses torchdata datapipes end-2-end.
     To avoid download at every epoch, we cache the data on-disk
     We do sanity check on dowloaded and extracted data