[`MPT`] Add  `require_bitsandbytes` on MPT integration tests
# What does this PR do?

As per title and as discussed offline with @ydshieh 
adding `require_bitsandbytes` is needed to avoid issues with past torch CI that doesn't have bnb installed on their Docker images.

cc @ydshieh 
_The documentation is not available anymore as the PR was closed or merged._<|||||>Jus out of curiosity does the `tooslow` decorator leads to tests still being run? 
```bash
    Slow tests are skipped while they're in the process of being fixed. No test should stay tagged as "tooslow" as
    these will not be tested by the CI.
```
Currently running MPT-7B is the only way to check if we are in sync with trust remote code weights, as it is the smallest model available<|||||>We also have `Salesforce/instructblip-vicuna-7b` but I haven't checked how long it takes to run on CI. So far it doesn't seems too problematic (those 7b models)<|||||>@ydshieh we also load that model using bnb: https://github.com/huggingface/transformers/blob/main/tests/models/instructblip/test_modeling_instructblip.py#L526 perhaps I can also add `require_bitsandbytes` there too<|||||>Yes, please. I missed that in the Past CI report. Thanks a lot!<|||||>No the `tooslow` tests are only run manually, not a runner.<|||||>Ok so I would say maybe we should keep testing mpt-7b so that we're aware of any potential issue through the daily CI 