rename "gpu" --> "device"
# What does this PR do?

Rename the arg names from "per_gpu" to "per_device" such that it aligns with the instruction in the readme  https://github.com/huggingface/transformers/tree/master/examples/text-classification#xnli
Could you run the code quality scripts for the code quality test? `make style && make quality`, after installing the latest code quality versions: `pip install -U .[quality]`<|||||>Looks like there is some styling issue. Could you run `make style` on your branch?<|||||>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.