Only use one gpu when generating the hidden states (text embeddings) by "Model output" api
### System Info

Report an issue about model output efficiency. 
I want to get the certain hidden states from the fine-tuned roberta model, then I chose Model Output api([link](https://huggingface.co/docs/transformers/main_classes/output#transformers.utils.ModelOutput)) to get those dense embeddings. 
However, when executing below model output code, only ONE gpu is used (actually, I have one worker with 4 gpus), which severely hinders inference efficiency. Also, I cannot find the instructions or docs to help to use multiple gpus when using model output.

<img width="1397" alt="Screen Shot 2022-07-27 at 15 51 37" src="https://user-images.githubusercontent.com/12815760/181194206-1483d4ce-39db-4f20-9355-a46844f6e39c.png">

<img width="681" alt="Screen Shot 2022-07-27 at 15 52 01" src="https://user-images.githubusercontent.com/12815760/181194264-893ceab2-c63e-4fa7-87c0-a9cf5b530782.png">



### Who can help?

_No response_

### Information

- [ ] The official example scripts
- [ ] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [ ] My own task or dataset (give details below)

### Reproduction

please refer to the above info form.

### Expected behavior

Multiple gpus can be used when using transformers Model Output api.
Hello, thanks for opening an issue! We try to keep the github issues for bugs/feature requests.
Could you ask your question on the [forum](https://discuss.huggingface.co) instead?

As an aside, we recommend looking into the [accelerate](https://github.com/huggingface/accelerate) package for multi-GPU usage.

Thanks!<|||||>Cannot loggin to hugging face, so just to pose the issue here.<|||||>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.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.