Fix and document Zero Shot Image Classification

[osanseviero]

What does this PR do?

This PR fixes 3 issues with zero-shot-image-classification

• It does not pick a default model, although there is one. This leads to weird error message

pipe = pipeline("zero-shot-image-classification")
>> ValueError: The task defaults can't be correctly selected. You probably meant "translation_XX_to_YY"

• The pipeline is not in the list at top of https://huggingface.co/docs/transformers/main_classes/pipelines
• The filter for available models has a typo, although right now this still leads to no models (ok just one).

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@Narsil @sgugger

[HuggingFaceDocBuilderDev]

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

[sgugger]

Thanks for fixing!

[Narsil]

LGTM