Allow DDPM scheduler to use model's predicated variance #132
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This is an implementation of the Improved Denoising Diffusion Probabilistic Models
which added the ability to learn the variance instead of just predicting the noise.
It is assumed that the step method receives a tensor shape (B, C, ...) when predicting noise.
At the moment, it only implements variance. In order to use it fully, the user needs to write the additional loss function from the paper into their training loop.
For future contribution, I think it would be beneficial to implement a losses module with different loss functions like the VB loss in the Improved Diffusion paper or other loss functions used in different diffusion papers (and maybe a trainer object). If needed, I can make new PRs with the relevant additions, or this can be an ongoing PR (the former seems better in case I'll stop contributing for some reason).