Improving `image-segmentation` pipeline tests.
# What does this PR do?

This PR (https://github.com/huggingface/transformers/pull/19367) introduced a few breaking changes:

- Removed an argument `mask_threshold`.
- Broke the default behavior (instance vs panoptic in the function call)
  https://github.com/huggingface/transformers/pull/19367/files#diff-60f846b86fb6a21d4caf60f5b3d593a04accb8f248de3029cccae2ff898c5bc3R119-R120
- Broke the actual masks: https://github.com/huggingface/transformers/pull/1961

This PR is the start of a handful that will aim at bringing back the old
behavior(s).

- tests should not have to specify `task` by default, unless we want to
  modify the behavior and have a lower form of segmentation running)
- `test_small_model_pt` should be working.

This specific PR starts with adding more information to the masks hash
because missing the actual mask was actual easy to miss (the hashes do
change, but it was easy to miss that one code path wasn't properly
updated).

So we go from a simple `hash` to
```
{"hash": #smaller hash, "shape": (h, w), "white_pixels": n}
```

The `shape` should help make sure the interpolation of the mask works
correctly, the `white_pixels` hopefully helps detect big regressions in
their amount when the hash gets modified.



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Fixes # (issue)


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_The documentation is not available anymore as the PR was closed or merged._<|||||>> as it seems this PR is adding supplementary information to existing tests. Do we still need to add a test for this missing codepath?

I meant that missing `astype(np.uint8)` was easy to miss, and since mask inspection was pretty manual, it was easy to check a few different setups, think everything was correct, and miss the one mask that was incorrect, and updated the tests without realizing the tests results were actually not making any sense.

This addition merely tries to reduce the risk of that happening, but if the tests don´t  use that path we won't know either.
