Onnx fix test
GPT2 `past_keys_values` format seems to have changed since last time I checked, now exporting for each layer tuple with 2 elements.

PyTorch's ONNX exporter doesn't seem to handle this format, so it was crashing with an error.

The PR assumes we don't currently support exporting `past_keys_values` for GPT2 and then disable the return of such values when constructing the model.

In order to support this behavior, `pipeline()` now ha a `model_kwargs: Dict[str, Any]` parameter which forwards the dict of parameters to model's `from_pretrained(..., **model_kwargs)`.  
Merging now to rebase the slow tests and re-run them.