Wrap VisualBERT integration test forward passes with torch.no_grad()
# What does this PR do?
As proposed in issue #14642, this PR wraps forward passes in VisualBERT integration tests with torch.no_grad(). This way, no unnecessary gradients are computed during inference.


## Before submitting
- [ ] This PR fixes a typo or improves the docs.
- [X] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/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/)?
- [ ] Did you make sure to update the documentation with your changes?
- [ ] Did you write any new necessary tests?


## Who can review?
@LysandreJik could you please take a look at it?
Thanks :)
_The documentation is not available anymore as the PR was closed or merged._