refactor: drop the Neuron RoPE path in Qwen Image

[JingyaHuang]

What does this PR do?

Reverts the Neuron-specific RoPE workaround added to Qwen-Image in #13718.

At the time, Neuron had no complex tensor support, so the RoPE freqs were sent to the device as rotation angles
(torch.angle) and applied with a real-valued cos/sin variant (apply_rotary_emb_qwen_neuron, dispatched through
ROPE_PER_DEVICE). The latest Neuron SDK runs complex tensors on device, in both eager and torch.compile, so
Qwen-Image can use the default complex RoPE on every device again. This removes:

• apply_rotary_emb_qwen_neuron and ROPE_PER_DEVICE
• the device.type == "neuron" branch in QwenEmbedRope._get_device_freqs and QwenEmbedLayer3DRope._get_device_freqs

The tensor-parallel changes from #13718 (per-head unflatten in the attention processor) are kept.

Same simplification as discussed in #14865 (comment) for
Qwen-Image-2.1.

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[sayakpaul]

/diffusers-bot pytest tests/models/transformers/test_models_transformer_qwenimage.py

[github-actions]

✅ pytest tests/models/transformers/test_models_transformer_qwenimage.py passed on GPU — view logs.