Fix audio_classifier_tutorial.py

Author: Will Feng

beginner_source/audio_classifier_tutorial.py

@@ -17,6 +17,7 @@
 import torch.nn.functional as F
 import torch.optim as optim
 from torchvision import datasets, transforms
+from torch.autograd import Variable
 from torch.utils.data import Dataset
 import torchaudio
 import pandas as pd