fix Qwen Image Transformer single file loading mapping function to be consistent with other loader APIs

[mbalabanski]

What does this PR do?

This PR changes the checkpoint_mapping_fn of the single_file loader of QwenImageTransformer2DModel to maintain a consistent API with the other loaders. Currently, the checkpoint mapping function is set to be a lambda which takes in an argument x and returns this same argument.

This PR fixes an issue caused by this lambda to use more consistent arguments as with the other checkpoint_to_diffusers functions.

Fixes #12891

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