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ecadcdef - [Bug] scheduling_ddpm: fix variance in the case of learned_range type. (#2090)

[Bug] scheduling_ddpm: fix variance in the case of learned_range type. (#2090) scheduling_ddpm: fix variance in the case of learned_range type. In the case of learned_range variance type, there are missing logs and exponent comparing to the theory (see "Improved Denoising Diffusion Probabilistic Models" section 3.1 equation 15: https://arxiv.org/pdf/2102.09672.pdf).