[tests] QwenImage Layer tests

[sayakpaul]

What does this PR do?

Adds tests for the newly added QwenImage layer model.

First, this PR fixes how we return the outputs; otherwise, we run into:

FAILED tests/pipelines/qwenimage/test_qwenimage_layered.py::QwenImageLayeredPipelineFastTests::test_callback_inputs - UnboundLocalError: local variable 'images' referenced before assignment

Second, we need to decide how to tackle this situation (which is fairly common across all failures):

FAILED tests/pipelines/qwenimage/test_qwenimage_layered.py::QwenImageLayeredPipelineFastTests::test_attention_slicing_forward_pass - TypeError: unsupported operand type(s) for -: 'list' and 'list'

Unlike other pipelines, we're always returning a list[Image.Image | np.ndarray | torch.Tensor]. One solution could be to apply np.stack() or torch.stack when output_type="np" or output_type="pt".

Cc: @yiyixuxu @naykun

[HuggingFaceDocBuilderDev]

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[sayakpaul]

A gentle ping @yiyixuxu

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