Change instr to outstr (PaddlePaddle#4558)

The first instr.strip is not use, change instr to outstr

Author: co63oc

Diff for: applications/question_answering/unsupervised_qa/finetune/question_generation/gen_utils.py

@@ -257,7 +257,7 @@ def post_process_sum(token_ids, tokenizer):
 def remove_template(instr):
     """Remove template prefix of decoded sequence."""
     outstr = instr.strip("问题：")
-    outstr = instr.strip("在已知答案的前提下，问题：")
+    outstr = outstr.strip("在已知答案的前提下，问题：")
     return outstr