Add Scheduler trait/enum about diffusers-rs

[mspronesti]

A possibility to solve the timestep type issue Is to have inside the trait a type, say timestep_t which requires to be cloned, copied and allowed to be converted to a primitive type

type timestep_t = Copy + Clone + toPrimitive;

and then use it in the trait as Self::timestep_t. Therefore, when implementing the trait for a particular scheduler one only needs to set it appropriately

impl Scheduler for MyScheduler {
    type timestep_t = usize;
}

I did all of this already, but I'm waiting to open a PR because I have 2 more pending and one more soon to be opened implementing the Heun Discrete scheduler 😅

[mspronesti]

On second thought, I'm not so sure this is a good idea. Not if we want to port other diffusion models, at least. In fact, some of the missing schedulers have a different implementation logic: some don't implement add_noise, some others have two different kind of steps (the predictive step and the correct step, with different return types), e.g. the stochastic differential equation (SDE) scheduler.