make the first diffusion step a first class property of LDM & Schedulers

[catwell]

Notes:

This changes the interface of SD in a backward-incompatible (but IMO simpler) way, as you can see from the changes in the README:

This also fixes #193.

[brycedrennan]

This is great.

I must confess I am tempted to try and make the __call__ method in the scheduler stateless but that's a large paradigm shift with questionable value.

    def __call__(
        self,
        x: Tensor,
        noise: Tensor,
        step: int,
        num_steps: int = 50,
        first_step: int = 0,
    ) -> Tensor:

[catwell]

I must confess I am tempted to try and make the call method in the scheduler stateless

It's actually what I had set out to do initially! But yeah I decided against it at least for now.