Update pipeline_ddpm.py

[KeepNoob]

Inspect the signature of the scheduler, then decide to whether to pass the generator as an augment to the self.scheduler.step()

What does this PR do?

Fixes huggingface#7354

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  PNDMScheduler Does Not Compatible with DDPMPipeline huggingface/diffusers#7354
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@yiyixuxu

[yiyixuxu]

Hi:
we didn't receive this PR at diffusers - can you send a PR to diffusers instead? see https://huggingface.co/docs/diffusers/en/conceptual/contribution#how-to-open-a-pr