Pass `model_kwargs` when loading a model in `pipeline()`
# What does this PR do?

Fixes #12448

When working on this, I considered adding a test, but none of the pipeline tests even use the `model_kwargs` parameter. I could add a test to one of the `test_pipelines_*.py` files to ensure that a local cache is used as a way of testing, but I'm not sure if that is the best option.

The parameter is indirectly used in `test_onnx.py`, but there is no verification that the parameter actually works properly. (Given that those tests pass without it working, maybe those tests might not need to use it, anyway.)

## 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?
- [x] 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.
- [ ] 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?

pipelines: @LysandreJik
I've written a test to ensure the `model_kwargs` parameter is being properly passed when loading a model.<|||||>Sorry about the failing style checks! I thought I ran them, but I guess not. They're hopefully good now.