Fixed error at DeformConvPack (open-mmlab#545)

Fixed error `TypeError: super(type, obj): obj must be an instance or subtype of type` .

mmdet/ops/dcn/modules/deform_conv.py

@@ -59,7 +59,7 @@ def forward(self, x, offset):
 class DeformConvPack(DeformConv):
 
     def __init__(self, *args, **kwargs):
-        super(ModulatedDeformConvPack, self).__init__(*args, **kwargs)
+        super(DeformConvPack, self).__init__(*args, **kwargs)
 
         self.conv_offset = nn.Conv2d(
             self.in_channels,