make MultipleDatasetIterator only if necessary (OpenNMT#1436)

onmt/train_single.py

@@ -99,15 +99,14 @@ def main(opt, device_id):
         opt, device_id, model, fields, optim, model_saver=model_saver)
 
     train_iterables = []
-    for train_id in opt.data_ids:
-        if train_id:
+    if len(opt.data_ids) > 1:
+        for train_id in opt.data_ids:
             shard_base = "train_" + train_id
-        else:
-            shard_base = "train"
-        iterable = build_dataset_iter(shard_base, fields, opt, multi=True)
-        train_iterables.append(iterable)
-
-    train_iter = MultipleDatasetIterator(train_iterables, device_id, opt)
+            iterable = build_dataset_iter(shard_base, fields, opt, multi=True)
+            train_iterables.append(iterable)
+        train_iter = MultipleDatasetIterator(train_iterables, device_id, opt)
+    else:
+        train_iter = build_dataset_iter("train", fields, opt)
 
     valid_iter = build_dataset_iter(
         "valid", fields, opt, is_train=False)