Extend modules to support more kinds of diffusion processes

[josephdviviano]

FYI: a natural extension would be using a different diffusion process (pinned Brownian motion -> Ornstein-Uhlenbeck process), which requires defining new forward and backward Estimators, while we can share the modules and the IsotropicGaussian distribution.

Originally posted by @hyeok9855 in #419 (comment)