Support loading base64 images in pipelines
Author: InventivetalentDev
Pull request: yes

# What does this PR do?

Adds support for loading base64-encoded images in pipelines. 
I primarily added this so it can be used by `transformers-cli serve`, since I found having to use an URL or reference to a local file a bit inconvenient for my use-case.

<!--
Congratulations! You've made it this far! You're not quite done yet though.

Once merged, your PR is going to appear in the release notes with the title you set, so make sure it's a great title that fully reflects the extent of your awesome contribution.

Then, please replace this with a description of the change and which issue is fixed (if applicable). Please also include relevant motivation and context. List any dependencies (if any) that are required for this change.

Once you're done, someone will review your PR shortly (see the section "Who can review?" below to tag some potential reviewers). They may suggest changes to make the code even better. If no one reviewed your PR after a week has passed, don't hesitate to post a new comment @-mentioning the same persons---sometimes notifications get lost.
-->


## 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?

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.

@Narsil 



Comments:
cc @amyeroberts 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_25633). All of your documentation changes will be reflected on that endpoint.

The PR looks good.

I'm uneasy about the actual format suggested, because some stuff are base64 decodable and yet real strings.
I feel like this should be in user code not really in `transformers`.

But I have the same feeling about loading images from URL so..

@Narsil Understood! As passing in strings is already supported for loading from url, I think this is an acceptable addition. Is there anything you'd like us to add to this PR to handle that? 

No not really.

It's choice. I'm not in favor, but I'm not super strongly opposed to it either.
Please choose which route you prefer.

@Narsil I'm inclined to agree. As we already accept strings, I'm going to merge. If continue support of this ends up being a headache (lots of code / lots of if/else logic) then we can think about ways to deprecate the support or reduce its scope.