Laplace Scheduler for DDPM

[gapatron]

What does this PR do?

This PR adds a Laplace noise scheduler that samples more frequently around mid-range noise levels (centered around log SNR = 0), following the formulation described in Improved Noise Schedule For Diffusion Training.

Improvements include:

• Improved sample quality (lower FID)
• Faster convergence during training
• Robust performance across resolution and loss objectives.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

Core library:

• Schedulers: @yiyixuxu

[hlky]

@bot /style

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[a-r-r-o-w]

@gapatron Thank you for the PR. The changes look good to me!

Could you run make fix-copies and push the relevant changes? Also, could you share some generation results and comparisons for anyone reviewing the PR to have something to look at immediately?

cc @asomoza