Pad updated (pytorch#683)

* pad updated

* checked pad

* pad checked

* pad checked

torchvision/transforms/functional.py

@@ -288,6 +288,12 @@ def pad(img, padding, fill=0, padding_mode='constant'):
         'Padding mode should be either constant, edge, reflect or symmetric'
 
     if padding_mode == 'constant':
+        if img.mode == 'P':
+            palette = img.getpalette()
+            image = ImageOps.expand(img, border=padding, fill=fill)
+            image.putpalette(palette)
+            return image
+
         return ImageOps.expand(img, border=padding, fill=fill)
     else:
         if isinstance(padding, int):
@@ -301,6 +307,14 @@ def pad(img, padding, fill=0, padding_mode='constant'):
             pad_right = padding[2]
             pad_bottom = padding[3]
 
+        if img.mode == 'P':
+            palette = img.getpalette()
+            img = np.asarray(img)
+            img = np.pad(img, ((pad_top, pad_bottom), (pad_left, pad_right)), padding_mode)
+            img = Image.fromarray(img)
+            img.putpalette(palette)
+            return img
+
         img = np.asarray(img)
         # RGB image
         if len(img.shape) == 3: