Fix translate.py bug (OpenNMT#1317) (OpenNMT#1318)

`translate.py` would ignore all sentences after processing
`shard_size*shard_size` sentences in `src`, if no `tgt` file
was provided. This commit fixes this.

translate.py

@@ -2,6 +2,8 @@
 # -*- coding: utf-8 -*-
 
 from __future__ import unicode_literals
+from itertools import repeat
+
 from onmt.utils.logging import init_logger
 from onmt.utils.misc import split_corpus
 from onmt.translate.translator import build_translator
@@ -17,7 +19,7 @@ def main(opt):
     translator = build_translator(opt, report_score=True)
     src_shards = split_corpus(opt.src, opt.shard_size)
     tgt_shards = split_corpus(opt.tgt, opt.shard_size) \
-        if opt.tgt is not None else [None]*opt.shard_size
+        if opt.tgt is not None else repeat(None)
     shard_pairs = zip(src_shards, tgt_shards)
 
     for i, (src_shard, tgt_shard) in enumerate(shard_pairs):