PBR-inspired Shading

## 🚀 Feature
Currently PyTorch3D supports basic shading methods (Phong, Gouraud etc). Although it mainly focuses on Rasterization scheme, there is still room for more realistic and PBR-inspired shading models.

## Motivation
Specular highlights are long-lived enemy for many techniques (e.g. photometric stereo), and PBR-inspired shading models can provide more realistic reflections, even with the rasterization scheme. Also, surface roughness can be embedded in shading.

## Pitch
I suggest choosing suitable PBR-inspired technique and implement as a shading option in PyTorch3D, and I’d like to help during the process with PR's. (ie: Cook-Torrance model might be good start)

(btw thank you @jcjohnson for your interest)