Implement `add_noise` in iPNDMScheduler

[pcuenca]

This method is missing from the recently-added IPNDMScheduler. It's not required for inference, but I think we should add it for consistency with all the others.

TODO:

• revert 56210ad when this is done.

[github-actions]

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[patrickvonplaten]

Honestly don't think it's that important iPNDMScheduler is only used for dance diffusion which is not heavily used.

IMO the "for consistency" argument is a dangerous one that might slow us down too much.

[pcuenca]

I agree, let's wait until we really need it. I just added this issue because a test failed, will remove the comment instead.