Fix naming issue with Image2TextGenerationPipeline
# What does this PR do?

Fixes naming issue with the Image2TextGenerationPipeline: naming was not consistent with other libraries.
See [this message](https://huggingface.slack.com/archives/C014N4749J9/p1662091141349169?thread_ts=1662048034.910319&cid=C014N4749J9). 

## 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/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] 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.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?

@Narsil, @sgugger 

_The documentation is not available anymore as the PR was closed or merged._