Validation and Evaluation not cumputed in run_qa.py
## Environment info
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     Don't forget to fill out the missing fields in that output! -->

- `transformers` version: 4.10.0.dev0
- Platform: ubuntu
- Python version:3.8

Models:

- roberta

Library:
- trainer: @sgugger
- pipelines: @LysandreJik


## Information

Model I am using  is Roberta.

* while running run_qa.py on squad 2.0 data there is missing training metrics, validation, and evaluation loss. 
* There is training loss and evaluation metric display after finetuning.

The tasks I am working on is:
* an official QuestionAnswer/Squad task


What command did you use to run the script? Setting `--eval_strategy epoch` for instance will give you the evaluation every epoch.<|||||>@sgugger Yes. 
At the time of evaluation [loss](https://github.com/huggingface/transformers/blob/75b8990d9068a2c6ef448c190f2595c17fbcb993/src/transformers/trainer.py#L2206) is empty<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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