[Fix] Fix SyncBN build in PyTorch 1.9 (#1138)

* [Fix] Fix SyncBN build in PyTorch 1.9

* fixed parrots SyncBN

mmcv/cnn/bricks/norm.py

@@ -106,7 +106,7 @@ def build_norm_layer(cfg, num_features, postfix=''):
     cfg_.setdefault('eps', 1e-5)
     if layer_type != 'GN':
         layer = norm_layer(num_features, **cfg_)
-        if layer_type == 'SyncBN':
+        if layer_type == 'SyncBN' and hasattr(layer, '_specify_ddp_gpu_num'):
             layer._specify_ddp_gpu_num(1)
     else:
         assert 'num_groups' in cfg_

mmcv/utils/parrots_wrapper.py

@@ -82,10 +82,6 @@ _AdaptiveAvgPoolNd, _AdaptiveMaxPoolNd, _AvgPoolNd, _MaxPoolNd = _get_pool()
 
 class SyncBatchNorm(SyncBatchNorm_):
 
-    def _specify_ddp_gpu_num(self, gpu_size):
-        if TORCH_VERSION != 'parrots':
-            super()._specify_ddp_gpu_num(gpu_size)
-
     def _check_input_dim(self, input):
         if TORCH_VERSION == 'parrots':
             if input.dim() < 2: