renderer: add support for rendering high dimensional textures for classification/segmentation use cases (#1248)

Summary:
For 3D segmentation problems it's really useful to be able to train the models from multiple viewpoints using Pytorch3D as the renderer. Currently due to hardcoded assumptions in a few spots the mesh renderer only supports rendering RGB (3 dimensional) data. You can encode the classification information as 3 channel data but if you have more than 3 classes you're out of luck.

This relaxes the assumptions to make rendering semantic classes work with `HardFlatShader` and `AmbientLights` with no diffusion/specular. The other shaders/lights don't make any sense for classification since they mutate the texture values in some way.

This only requires changes in `Materials` and `AmbientLights`. The bulk of the code is the unit test.

Pull Request resolved: https://github.com/facebookresearch/pytorch3d/pull/1248

Test Plan: Added unit test that renders a 5 dimensional texture and compare dimensions 2-5 to a stored picture.

Reviewed By: bottler

Differential Revision: D37764610

Pulled By: d4l3k

fbshipit-source-id: 031895724d9318a6f6bab5b31055bb3f438176a5