master-b6899e8

feat: add Euler, Heun and DPM++ (2M) samplers (#50)

* Add Euler sampler

* Add Heun sampler

* Add DPM++ (2M) sampler

* Add modified DPM++ (2M) "v2" sampler.

This was proposed in a issue discussion of the stable diffusion webui,
at https://github.com/AUTOMATIC1111/stable-diffusion-webui/discussions/8457
and apparently works around overstepping of the DPM++ (2M) method with
small step counts.

The parameter is called dpmpp2mv2 here.

* match code style

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Co-authored-by: Urs Ganse <urs@nerd2nerd.org>
Co-authored-by: leejet <leejet714@gmail.com>