[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).