process the case that no weight in the module (#153)

Signed-off-by: han_hans <han_hans@apple.com>

mmcv/cnn/weight_init.py

@@ -2,7 +2,8 @@ import torch.nn as nn
 
 
 def constant_init(module, val, bias=0):
-    nn.init.constant_(module.weight, val)
+    if hasattr(module, 'weight') and module.weight is not None:
+        nn.init.constant_(module.weight, val)
     if hasattr(module, 'bias') and module.bias is not None:
         nn.init.constant_(module.bias, bias)