ONNX export: move sample input to same device as model when inferring shapes
# What does this PR do?

Training a model on a GPU and exporting it afterwards to ONNX has raised a `RuntimeError`, because the model and the [sample input](https://github.com/huggingface/transformers/blob/1a3e0c4fe6868b4eb1105dfe601a79d7e5d11a0f/src/transformers/convert_graph_to_onnx.py#L196) in [`transformers.convert_graph_to_onnx.infer_shapes()`](https://github.com/huggingface/transformers/blob/1a3e0c4fe6868b4eb1105dfe601a79d7e5d11a0f/src/transformers/convert_graph_to_onnx.py#L161-L222) were not on the same device. This PR moves the output to the same device as the model.

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests) Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

@mfuntowicz (according to `git blame`)

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/master/CONTRIBUTING.md) are likely to be ignored.