Replace unknown word by original source word when empty string is given (#770)

Summary:
Pull Request resolved: https://github.com/pytorch/fairseq/pull/770

Without this change comment here https://fburl.com/w1cejgw9 is inconsistent with the implementation.

Reviewed By: xianxl

Differential Revision: D15582826

fbshipit-source-id: 16d8368560153b251beed8b290f51fcdd8a8faee

fairseq/utils.py

@@ -84,7 +84,7 @@ def set_incremental_state(module, incremental_state, key, value):
 def load_align_dict(replace_unk):
     if replace_unk is None:
         align_dict = None
-    elif isinstance(replace_unk, str):
+    elif isinstance(replace_unk, str) and len(replace_unk) > 0:
         # Load alignment dictionary for unknown word replacement if it was passed as an argument.
         align_dict = {}
         with open(replace_unk, 'r') as f: