Add experimental Heun scheduler

[pcuenca]

Without duplicating the sigmas and timesteps (reference: #1336)

➕ No duplication of sigmas and timesteps. Does not require ugly function index_for_timestep (or confusing oneliner repeated twice in the code)

➖ Requires modification of all pipelines. The pipeline loop is uglier.

All in all I don't think we gain any more clarity than in #1336, so I recommend we adopt #1336 unless it's important that the timesteps and sigmas are not duplicated.

This implementation does work. I'll push a fix for #1336 so it works too.

[HuggingFaceDocBuilderDev]

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