Enabling MaskFormer in pipelines
Author: Narsil
Pull request: yes

# What does this PR do?

Enabling MaskFormer in pipelines.


```python
pipeline = pipeline(
    model=MaskFormerForInstanceSegmentation.from_pretrained(".."),
    feature_extractor=AutoFeatureExtractor.from_pretrained("..."),
    task="image-segmentation",
)
```

However `pipeline(model="...")` doesn't work yet:
- We need to add `pipeline_tag` on the model hub for it to work
- We need to add `MaskFormerForInstanceSegmentation` to `MODEL_FOR_INSTANCE_SEGMENTATION_MAPPING` for the automated tests on random models to work



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Fixes # (issue)


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Comments:
I'll go ahead and merge this, and create another PR for the dict.