fix syncbn when dist is unavailabel (#388)

mmcv/ops/sync_bn.py

@@ -109,7 +109,9 @@ class SyncBatchNormFunction(Function):
             None, None, None, None
 
 
-class SyncBatchNorm(Module):
+if dist.is_available():
+
+    class SyncBatchNorm(Module):
 
         def __init__(self,
                      num_features,
@@ -175,8 +177,8 @@ class SyncBatchNorm(Module):
 
             if self.training or not self.track_running_stats:
                 return SyncBatchNormFunction.apply(input, self.running_mean,
-                                               self.running_var, self.weight,
-                                               self.bias,
+                                                   self.running_var,
+                                                   self.weight, self.bias,
                                                    exponential_average_factor,
                                                    self.eps, self.group,
                                                    self.group_size)
@@ -194,3 +196,12 @@ class SyncBatchNorm(Module):
             s += f'track_running_stats={self.track_running_stats}, '
             s += f'group_size={self.group_size})'
             return s
+
+else:
+
+    class SyncBatchNorm(Module):
+
+        def __init__(self, *args, **kwargs):
+            raise NotImplementedError(
+                'SyncBatchNorm is not supported in this OS since the '
+                'distributed package is not available')