Fix img seg tests (load checkpoints from hf-internal-testing) (#17939)

* Revert "Skip failing test until they are fixed."

This reverts commit 8f400775fc.

* Use `tiny-detr` checkpts from `hf-internal-testing`

tests/pipelines/test_pipelines_image_segmentation.py

@@ -146,10 +146,9 @@ class ImageSegmentationPipelineTests(unittest.TestCase, metaclass=PipelineTestCa
     def test_small_model_tf(self):
         pass
 
-    @unittest.skip("Model has moved, skip until it's fixed.")
     @require_torch
     def test_small_model_pt(self):
-        model_id = "mishig/tiny-detr-mobilenetsv3-panoptic"
+        model_id = "hf-internal-testing/tiny-detr-mobilenetsv3-panoptic"
 
         model = AutoModelForImageSegmentation.from_pretrained(model_id)
         feature_extractor = AutoFeatureExtractor.from_pretrained(model_id)