Fix device in longformer onnx path
# What does this PR do?

Longformer has a custom path in its `_chunk()` method, in order to be tracable (to some extent) + exportable to ONNX. https://github.com/huggingface/transformers/pull/20292 fixed a bug where this special path was always registering a non-general case:

https://github.com/huggingface/transformers/blob/9ef46659da45f6b605873ca59124d03976990b33/src/transformers/models/longformer/modeling_longformer.py#L785-L787

It seems that the `else` path that should be taken in the export was actually never tested, and notably never tested on GPU. This PR fixes a device assignment.

The test `RUN_SLOW=1 python -m pytest -v tests/onnx/test_onnx_v2.py::OnnxExportTestCaseV2::test_pytorch_export_on_cuda_102_longformer_token_classification` is now running fine.

## Who can review?

@lewtun @ydshieh 
gently pinging @sgugger for final approval :)<|||||>_The documentation is not available anymore as the PR was closed or merged._