Support loading base64 images in pipelines

[InventivetalentDev]

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.

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@Narsil

[ArthurZucker]

cc @amyeroberts

[amyeroberts]

Thanks for adding this!

Really useful functionality - main comment is to update all logic of the function e.g. errors raised and make it a bit more defensive.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.

[amyeroberts]

Thanks for adding this!

cc @Narsil for a second opinion on the changes for the pipeline

[Narsil]

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..

[amyeroberts]

@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?

[Narsil]

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.

[amyeroberts]

@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.