Add USAM module

Author: layumi

model.py

@@ -46,8 +46,8 @@ def __init__(self, kernel_size=3, padding=1, polish=True):
 
     def __call__(self, x):
         fmap = x.sum(1, keepdim=True)      
-        x1 = F.conv2d(fmap, self.weight, padding=self.pad)
-        x2 = F.conv2d(fmap, self.weight2, padding=0) 
+        x1 = nn.functional.conv2d(fmap, self.weight, padding=self.pad)
+        x2 = nn.functional.conv2d(fmap, self.weight2, padding=0) 
 
         att = x2 - x1
         att = self.bn(att)