Use variable instead of magic number in data_parallel_tutorial.py (pytorch#244)

Author: fthoele

beginner_source/blitz/data_parallel_tutorial.py

@@ -79,7 +79,7 @@ def __getitem__(self, index):
     def __len__(self):
         return self.len
 
-rand_loader = DataLoader(dataset=RandomDataset(input_size, 100),
+rand_loader = DataLoader(dataset=RandomDataset(input_size, data_size),
                          batch_size=batch_size, shuffle=True)