fix backcompatibility in translator

Author: francoishernandez

onmt/translate/translator.py

@@ -316,7 +316,8 @@ def translate(
             [('src', src_data), ('tgt', tgt_data)])
 
         # corpus_id field is useless here
-        self.fields.pop('corpus_id')
+        if self.fields.get("corpus_id", None) is not None:
+            self.fields.pop('corpus_id')
         data = inputters.Dataset(
             self.fields, readers=_readers, data=_data, dirs=_dir,
             sort_key=inputters.str2sortkey[self.data_type],