Add PNDM scheduler

[mspiegel]

Adds support for the PNDM scheduler. It is the default scheduler in "NOP's Stable Diffusion" notebook. I don't pretend to understand the math or have any training as an artist, but PNDM seems to generate 'better' results in my art, for some subjective definition of better. Closes #165

[lkwq007]

Thanks for adding the PNDM scheduler