Update CLIPFeatureExtractor to convert PIL image to RGB
# What does this PR do?

<!--

- Converts PIL image to `RGB` if processed by CLIPFeatureExtractor.

-->

Currently PIL images with `RGBA` format throws an error when being processed by CLIPFeatureExtractor. 

CLIPFeatureExtractor.normalize() throws the following error:
`.../sentence-transformer/lib/python3.9/site-packages/transformers/image_utils.py", line 185, in normalize
    return (image - mean) / std
ValueError: operands could not be broadcast together with shapes (4,224,224) (3,) `

The original [clip model preprocesses PIL Images](https://github.com/openai/CLIP/blob/main/clip/clip.py#L74) by converting all PIL images into `RGB` format.


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag members/contributors who may be interested in your PR.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15886). All of your documentation changes will be reflected on that endpoint.<|||||>@patil-suraj @sgugger any idea why the `Build dev documentation / build_and_package` check failed? based on the error log, I initially suspected that master branch is out of sync (i.e. there are new commits), however, I just verified that it is up to date 🤔<|||||> You can rebase to master to solve the failing test.<|||||>@patil-suraj I checkout my branch and ran `git rebase master`, it returns `Current branch convert-pil-image-to-rgb-clip-model is up to date.` <|||||>You should rebase with `huggingface/transformers`, for example if your remote is called `upstream` you can run the following commands to rebase.

```bash
git fetch upstream
git rebase upstream/master
``` 

and then push.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.<|||||>Hey @hengkuanwee , this slipped through the cracks. Do you maybe want to open new PR for this ?<|||||>hi @patil-suraj, sure i'll open a new PR for this!