PRX Set downscale_freq_shift to 0 for consistency with internal implementation

[DavidBert]

What does this PR do?

Realized there was a discrepancy between our internal timestep embedding and the version used in Diffusers.
Setting downscale_freq_shift = 0 aligns both implementations and makes the embeddings fully equivalent.

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Who can review?

@sayakpaul, @yiyixuxu and @asomoza

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[yiyixuxu]

thanks!

[HuggingFaceDocBuilderDev]

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