[PL examples]: fix progress bar bug
As shown by @prabalbansal in #3576, the fine tuning script for seq2seq models is failing with the following error:

![](https://user-images.githubusercontent.com/30004110/78461843-b2006e00-76cc-11ea-9be8-06171cad86e5.png)

This PR includes the fix suggested by @sshleifer in [this comment](https://github.com/huggingface/transformers/issues/3576#issuecomment-611755174), which made it work, but it seems to be suboptimal since the loss is not shown in the progress.
This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
