Implement Heun discrete scheduler

[mspronesti]

This PR aims at integrating Heun Discrete Scheduler into this repository, solving the fourth task metioned in #23 .

This implementation ports all the features supported in the official HF implementation. A couple of observations:

• to run the examples with this scheduler, one needs to generalize them first (see #35), as mentioned for the past 2 schedulers I implemented
• the number of inference steps is not actually equal to the value the user chooses for this hyperparameter, as this scheduler requires to first compute a linspace drawing num_inference_steps samples and then to concatenate the first element with the interleaved repetitions of the timesteos tensor (2 times), for a total of 2 * num_inference_steps - 1 timestpes (see here). Thereby, if you see, running one of the pipelines something like 58/30 that's normal, it's just that the println!() in the example needs to be fixed.

[mspronesti]

One of the workflows (windows-2019 stable) took 40 mins to pass. Not sure if anything weird happened on GitHub side 😅 I recommend re-running it, just to make sure, but I think it was just accidental.

EDIT: I confirm it. I mirrored my branch and run the action. All the checks passed in ~5 mins as expected.

[LaurentMazare]

Thanks for the PR!