Fix circle ci flaky fail of wmt example 
Weird bug which might be fixed by forcing bleu scorer. 

Test pass locally, but seem to fail on circle ci.
This is from this right? https://circleci.com/gh/huggingface/transformers/26455?utm_campaign=workflow-failed&utm_medium=email&utm_source=notification<|||||>Yeah exactly - it should be fixed now. 
I will push a slightly cleaner version in a second :-) <|||||>In general, every example test that creates folder or files should deleted them afterwards to avoid same file naming collisions. Will open a PR about this. @julien-c @sshleifer @LysandreJik 