[skip ci] make LSUN OS agnostic (#3455)

Co-authored-by: vfdev <vfdev.5@gmail.com>
Co-authored-by: Francisco Massa <fvsmassa@gmail.com>

torchvision/datasets/lsun.py

@@ -83,7 +83,7 @@ class LSUN(VisionDataset):
         self.dbs = []
         for c in self.classes:
             self.dbs.append(LSUNClass(
-                root=root + '/' + c + '_lmdb',
+                root=os.path.join(root, f"{c}_lmdb"),
                 transform=transform))
 
         self.indices = []