fix custom diffusion tests

[DN6]

What does this PR do?

Fixes failing model test with custom diffusion attention processor

FAILED tests/models/test_models_unet_2d_condition.py::UNet2DConditionModelTests::test_custom_diffusion_save_load - RuntimeError: Expected all tensors to be on the same device, but found at least two devices, cuda:0 and cpu! (when checking argument for argument mat2 in method wrapper_CUDA_mm)

Fixes # (issue)

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[patrickvonplaten]

Thanks!