Text conditional DPM

[AkioHayakawa-sony]

This PR includes following items to support Photorealistic Text-to-Image Diffusion Models with Deep Language Understanding (AKA Imagen).

Training script

• text conditional training with LAION dataset

Generation script

• Inference with Classifier-Free Guidance

Model arch.

• text embedding and cross attention
• Efficient UNet
• Gaussian Conditioning Augmentation for robust upsampler
• numerical stability improvement