Update train_semi.py

Added "net" to cfg for making this train_semi.py detect

Author: ksh3490

train_semi.py

@@ -85,7 +85,7 @@ def main():
     else:
         modules_head = [model.decoder]
 
-    if cfg.get("sync_bn", True):
+    if cfg["net"].get("sync_bn", True):
         model = torch.nn.SyncBatchNorm.convert_sync_batchnorm(model)
 
     model.cuda()