DDPM pretrained model

Try using MegDiffuson to sample images with DDPM pretrained model. (It's still a demo script now.)

python3 -m megdiffusion.pipeline.ddpm.sample --config ./configs/ddpm/cifar10.yaml

Other configs are available here.

Thanks to @Asthestarsfalll for converting original Tensorflow checkpoints to MegEngine (from @pesser ‘s PyTorch pretrained diffusion models and pretrained models on Huggingface provided by Google ).