Update image segmentation pipeline test

[amyeroberts]

What does this PR do?

The image segmentation pipeline tests - tests/pipelines/test_pipelines_image_segmentation.py - were failing after the merging of #1849 (49e44b2). This was due to the difference in rescaling.

Previously the images were rescaled by image = image / 255. In the new commit, a rescale method was added, and images rescaled using image = image * scale. This was known to cause small differences in the processed images (see
PR comment).

Testing locally, changing the rescale method to divide by a scale factor (255) resulted in the tests passing. It was therefore decided after discussion with @ydshieh the test values could be updated, as there was no logic difference between the commits.

Comparing masks

The same code as in the segmentation pipeline was run to compare the output masks.

#git_hash = "86d0b26d6c5566506b1be55002654b48d9c19ffe"
git_hash = "49e44b216b2559e34e945d5dcdbbe2238859e29b"

from transformers import pipeline
import numpy as np
model_id = "facebook/detr-resnet-50-panoptic"
image_segmenter = pipeline("image-segmentation", model=model_id)
outputs = image_segmenter("http://images.cocodataset.org/val2017/000000039769.jpg")
for i, o in enumerate(outputs):
    img = o['mask']
    np.save(f"img_{git_hash}_{i}", img)
    img.save(f"img_{git_hash}_{i}.png")

Mask generate for output 0 on commit hash 86d0b26 (parent commit)

Mask generate for output 0 on commit hash 49e44b2

Checking the pixel values, there was a single pixel difference

>>> git_hash_1 = "86d0b26d6c5566506b1be55002654b48d9c19ffe"
>>> git_hash_2 = "49e44b216b2559e34e945d5dcdbbe2238859e29b"
>>> 
>>> def max_diff(a, b):
>>>     return np.amax(np.abs(a - b))
>>> 
>>> for i in range(6):
>>>     print(f"\nComparing mask {i}")
>>>     img_orig = f"img_{git_hash_1}_{i}"
>>>     img_new = f"img_{git_hash_2}_{i}"
>>>     arr_orig = np.load(img_orig + ".npy")
>>>     arr_new = np.load(img_new + ".npy")
>>>     print(f"Max difference: ", max_diff(arr_orig, arr_new))
>>> 
>>>     m = arr_orig != arr_new
>>>     # image shape is 480 x 640
>>>     x = np.arange(480 * 640).reshape(480, 640)
>>>     print("No. pixels different: ", len(x[m]))

Output:

Comparing mask 0
Max difference:  255
No. pixels different:  1

Comparing mask 1
Max difference:  0
No. pixels different:  0

Comparing mask 2
Max difference:  0
No. pixels different:  0

Comparing mask 3
Max difference:  0
No. pixels different:  0

Comparing mask 4
Max difference:  0
No. pixels different:  0

Comparing mask 5
Max difference:  0
No. pixels different:  0

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?

[HuggingFaceDocBuilderDev]

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

[ydshieh]

LVGTM, thank you @amyeroberts for checking all the details!

[ydshieh]

@amyeroberts Let's merge this PR?

[sgugger]

Amy is on vacation ;-)