PLY pointcloud loading

Summary:
Allow PLY files to not contain faces. Allow loading pointclouds with color, at least encoded according to the way of some cloudcompare examples.

TODO: Allow vertex normals to be read, and allow vertex colors to be written. Make the return type of load_ply something more user friendly, like a dict.

Noticed in #209

Reviewed By: nikhilaravi

Differential Revision: D22573314

fbshipit-source-id: 72ba1f7c6417f5dfc83f2ebf359eff017057635c

Author: bottler

pytorch3d/io/pluggable.py

@@ -12,7 +12,7 @@
 
 from .obj_io import MeshObjFormat
 from .pluggable_formats import MeshFormatInterpreter, PointcloudFormatInterpreter
-from .ply_io import MeshPlyFormat
+from .ply_io import MeshPlyFormat, PointcloudPlyFormat
 
 
 """
@@ -74,6 +74,7 @@ def __init__(
     def register_default_formats(self) -> None:
         self.register_meshes_format(MeshObjFormat())
         self.register_meshes_format(MeshPlyFormat())
+        self.register_pointcloud_format(PointcloudPlyFormat())
 
     def register_meshes_format(self, interpreter: MeshFormatInterpreter) -> None:
         """