恢复ctx转换中的代码，tests中对device的处理在cpu和cpu:0时判断为非同一设备

python/dgl/backend/pytorch/tensor.py

@@ -89,18 +89,12 @@ def device_id(ctx):
     else:
         return ctx.index
 
-__devtype_th_map = {
-    1: "cpu",
-    2: "cuda",  # cuda device
-    10: "cuda"  # rocm device
-}
-
 def to_backend_ctx(dglctx):
-    
     dev_type = dglctx.device_type
-    if dev_type in __devtype_th_map:
-        th_type = __devtype_th_map[dev_type]
-        return th.device(th_type, dglctx.device_id)
+    if dev_type == 1:
+        return th.device('cpu')
+    elif dev_type == 2 or dev_type == 10:
+        return th.device('cuda', dglctx.device_id)
     else:
         raise ValueError('Unsupported DGL device context:', dglctx)