ODE sampling get unrealistic images

[yuanzhi-zhu]

Thanks for your great work which inspires so many!!

As mentioned in #13, I also noticed that the pure ODE sampling results is not satisfying for 256*256 images (maybe also bigger size).

Most of the time these generated images are blurry or over-smooth, sometimes even very noisy.

To reproduce, one can simply run the demo notebook with pretrained checkpoints provided by the authors.

[yuanzhi-zhu]

I suspect the reason is ODE solver accumulates more error for larger image (data size)