fix image show error (pytorch#165)

* since it normalized the channel value to specific std and mean, when show the image and  reverse its value to [0, 1], sometimes it got error of **Floating point image RGB values must be in the 0..1 range.**

* use np.clip instead

Author: tztztztztz

beginner_source/transfer_learning_tutorial.py

@@ -111,6 +111,7 @@ def imshow(inp, title=None):
     mean = np.array([0.485, 0.456, 0.406])
     std = np.array([0.229, 0.224, 0.225])
     inp = std * inp + mean
+    inp = np.clip(inp, 0, 1)
     plt.imshow(inp)
     if title is not None:
         plt.title(title)