Add the ImageClassificationPipeline
This PR adds the `ImageClassificationPipeline`. It is tested on DeiT and ViT and should enable the inference API for these models.

Since I encountered an issue with the `AutoExtractor`, I fixed it as seen with @sgugger (namely switched it from using names such as `"deit"` to using the configuration class as a key, similarly to what we do in tokenizers and the other auto classes.

Please let me know if you would like me to split this PR into multiple PRs for simpler reviews, happy to do so.

@NielsRogge, happy to have your review.
LGTM, thank you for adding this.<|||||>I should have addressed all comments. 

@Narsil, if you could review once again when you have time, would love your feedback on the updated tests and on the updated `__init__` method.