feat: add Euler, Heun and DPM++ (2M) samplers (leejet#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 AUTOMATIC1111/stable-diffusion-webui#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>

feat: add option to switch the sigma schedule (leejet#51)

Concretely, this allows switching to the "Karras" schedule from the
Karras et al 2022 paper, equivalent to the samplers marked as "Karras"
in the AUTOMATIC1111 WebUI. This choice is in principle orthogonal to
the sampler choice and can be given independently.

feat: set default rng to cuda

feat: add CUDA RNG

fix: insufficient memory error on macOS

fix: seed should be 64 bit

use new graph api to avoid stack overflow on msvc

feat: add SD2.x support (leejet#40)

fix: do not check weights of open clip last layer

fix: correct the handling of weight loading