Bert Abs not using GPU
# 🐛 Bug

## Information

Model I am using: Bert abs (https://github.com/huggingface/transformers/tree/master/examples/seq2seq/bertabs)
-> remi/bertabs-finetuned-cnndm-extractive-abstractive-summarization model

Language I am using the model on: English

The problem arises when using:
* [x] the official example scripts: (give details below)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [x] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1. Install Conda Environment with proper pytorch installation
2. Verify that `torch.cuda.is_available()` return True
3. Start run_summarization.py as stated in the Readme

## Expected behavior

I would expect, that the models uses GPU for Training / Generation. Instead I have a very high CPU usage, Gpu usage is nearly zero and GPU clock stays idle. Since I verified that the GPU and cuda is available, I expect it to use it.

--no_cuda is not set, so the default of False is taken used

## Environment info

- `transformers` version: 2.11.0
- Platform: Windows-10-10.0.18362-SP0
- Python version: 3.7.7
- PyTorch version (GPU?): 1.5.1 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: Yes
- Using distributed or parallel set-up in script?: No


This can be resolved by not using the remi/bertabs-finetuned-cnndm-extractive-abstractive-summarization model but the https://huggingface.co/remi/bertabs-finetuned-extractive-abstractive-summarization. With cnndm prefix I would expect it as a gpu version, but the opposite is the case.
Changing the model fixes the error