Accumulate points (#4)

Summary:
Code for accumulating points in the z-buffer in three ways:
1. weighted sum
2. normalised weighted sum
3. alpha compositing

Pull Request resolved: https://github.com/fairinternal/pytorch3d/pull/4

Reviewed By: nikhilaravi

Differential Revision: D20522422

Pulled By: gkioxari

fbshipit-source-id: 5023baa05f15e338f3821ef08f5552c2dcbfc06c

tests/test_face_areas_normals.py

@@ -76,7 +76,10 @@ class TestFaceAreasNormals(TestCaseMixin, unittest.TestCase):
         verts_torch = verts.detach().clone().to(dtype)
         verts_torch.requires_grad = True
         faces_torch = faces.detach().clone()
-        areas_torch, normals_torch = TestFaceAreasNormals.face_areas_normals_python(
+        (
+            areas_torch,
+            normals_torch,
+        ) = TestFaceAreasNormals.face_areas_normals_python(
             verts_torch, faces_torch
         )
         self.assertClose(areas_torch, areas, atol=1e-7)