lower default batch producer queue size (#1716)

Author: pltrdy

onmt/opts.py

@@ -375,7 +375,7 @@ def train_opts(parser):
               help="IP of master for torch.distributed training.")
     group.add('--master_port', '-master_port', default=10000, type=int,
               help="Port of master for torch.distributed training.")
-    group.add('--queue_size', '-queue_size', default=400, type=int,
+    group.add('--queue_size', '-queue_size', default=40, type=int,
               help="Size of queue for each process in producer/consumer")
 
     group.add('--seed', '-seed', type=int, default=-1,