use 'cpu' instead of None for Iterator (#745)

Author: avinashsai

torchtext/data/iterator.py

@@ -3,6 +3,7 @@
 
 import logging
 
+import torch
 from .utils import RandomShuffler
 from .batch import Batch
 from .dataset import Dataset
@@ -65,6 +66,12 @@ def __init__(self, dataset, batch_size, sort_key=None, device=None,
                            + " or passing a string as an argument. This behavior will be"
                            + " deprecated soon and currently defaults to cpu.")
             device = None
+
+        if device is None:
+            device = torch.device('cpu')
+        elif isinstance(device, str):
+            device = torch.device(device)
+
         self.device = device
         self.random_shuffler = RandomShuffler()