Fully Bayesian Matern GP with dimension scaling prior (pytorch#2855)

Summary:
Pull Request resolved: pytorch#2855

See title. This also does a sizable refactor to the fully bayesian GPs to share code where possible.

Reviewed By: hvarfner

Differential Revision: D74824586

fbshipit-source-id: e58146fa33e9dc7ba850b28989fc28c7c0be94e4

Author: sdaulton