fully bayesian linear model (#2697)

Summary:
Pull Request resolved: #2697

Implements a Fully Bayesian linear model with a fully bayesian treatment of input warping (optional).

Reviewed By: saitcakmak

Differential Revision: D68529442

fbshipit-source-id: 79214be26df9a318740fcc3a4f18c8df441afab2

Author: sdaulton