Fix test_auto_backbone_timm_model_from_pretrained

[ydshieh]

What does this PR do?

This test assert model.out_indices, (-2, -1) and it was written on 2023/12.

However, we have

out_indices = list(out_indices) if out_indices is not None else None

in src/transformers/utils/backbone_utils.py which is written on 2024/05

It's just time to update the test so we don't see it fails forever.

[HuggingFaceDocBuilderDev]

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