[Bug] Avoid recursion error when using customized norm layer without _abbr_ (#732)

* avoid recursion error when using customized norm layer without _abbr_

* fix unittest bug

mmcv/cnn/bricks/norm.py

@@ -65,7 +65,7 @@ def infer_abbr(class_type):
         elif 'instance' in class_name:
             return 'in'
         else:
-            return 'norm'
+            return 'norm_layer'
 
 
 def build_norm_layer(cfg, num_features, postfix=''):

tests/test_cnn/test_build_layers.py

@@ -101,7 +101,7 @@ def test_infer_norm_abbr():
     class FancyNorm:
         pass
 
-    assert infer_norm_abbr(FancyNorm) == 'norm'
+    assert infer_norm_abbr(FancyNorm) == 'norm_layer'
 
 
 def test_build_norm_layer():