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 <[email protected]>
Co-authored-by: leejet <[email protected]>
