Experimental glTF reading

Summary: Experimental data loader for taking the default scene from a GLB file and converting it to a single mesh in PyTorch3D.

Reviewed By: nikhilaravi

Differential Revision: D25900167

fbshipit-source-id: bff22ac00298b83a0bd071ae5c8923561e1d81d7

Author: bottler

docs/notes/io.md

@@ -22,3 +22,13 @@ and to save a pointcloud you might do
 pcl = Pointclouds(...)
 IO().save_point_cloud(pcl, "output_pointcloud.obj")
 ```
+
+For meshes, this supports OBJ, PLY and OFF files.
+
+For pointclouds, this supports PLY files.
+
+In addition, there is experimental support for loading meshes from
+[glTF 2 assets](https://github.com/KhronosGroup/glTF/tree/master/specification/2.0)
+stored either in a GLB container file or a glTF JSON file with embedded binary data.
+This must be enabled explicitly, as described in
+`pytorch3d/io/experimental_gltf_io.ply`.