Expose AutoModelForMaskGeneration
As per title, with this PR `from transformers import AutoModelForMaskGeneration` works.

An alternative could be to remove `AutoModelForMaskGeneration` (as `AutoModel` already does the job), but currently for sam Hub metadata it is `AutoModelForMaskGeneration` that is used and not `AutoModel`: https://huggingface.co/datasets/huggingface/transformers-metadata/blob/main/pipeline_tags.json#L576
_The documentation is not available anymore as the PR was closed or merged._<|||||>Either way is fine for me - it's just to have consistency between the transformers Hub metadata and transformers. An either fix would be to just change the metadata, and remove the `AutoModelForMaskGeneration`.<|||||>So which one do you want @ArthurZucker ? I'm fine either way.<|||||>Let's go with `AutoModelForMaskGeneration` 😉 