No easy support for custom noise priors

[Aniket1998]

The loss-ops and sampling from the noise priors are too entangled with one another, and the entire library is written keeping a Normal Distribution in mind for the noise prior. This leads to inconveniences such as overriding the entire train ops of a loss (and even the sampler) just to change something as minor as changing the noise prior from Normal to Uniform.

I propose having a separate set of modules for priors, keeping both fixed and learnable priors in mind. Will submit a PR regarding the same.