Wrote extended DDIM scheduler that takes pairs of timesteps

[lukovnikov]

Re-implemented #1236 a bit differently after a suggestion from @patrickvonplaten . Now the original DDIMScheduler is untouched but a new class is introduced with extra featuers (see below) and DDIMPipeline is slightly change to automatically accommodate the alternative DDIM scheduler implementation.

• changed timestep selection such that it is more uniform if number of sampling steps doesn't cleanly divide number of training steps (this leads to not evenly spaced jumps)
• changed implementation of step() to use pairs (x_tm1, x_t) in order to support the previous point and possibly non-linear substep selections (like the quadratic one from the appendix of the DDIM paper)

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.

[github-actions[bot]]

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