🚨 out_indices always a list (#30941)

* out_indices always a list

* Update src/transformers/utils/backbone_utils.py

* Update src/transformers/utils/backbone_utils.py

* Move type casting

* nit

tests/models/timm_backbone/test_modeling_timm_backbone.py

@@ -131,7 +131,7 @@ class TimmBackboneModelTest(ModelTesterMixin, BackboneTesterMixin, PipelineTeste
         # Out indices are set to the last layer by default. For timm models, we don't know
         # the number of layers in advance, so we set it to (-1,), whereas for transformers
         # models, we set it to [len(stage_names) - 1] (kept for backward compatibility).
-        self.assertEqual(timm_model.out_indices, (-1,))
+        self.assertEqual(timm_model.out_indices, [-1])
         self.assertEqual(transformers_model.out_indices, [len(timm_model.stage_names) - 1])
 
         timm_model = AutoBackbone.from_pretrained(timm_checkpoint, use_timm_backbone=True, out_indices=[1, 2, 3])