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

docs/tutorials/deform_source_mesh_to_target_mesh.ipynb

@@ -673,9 +673,9 @@
    "provenance": []
   },
   "kernelspec": {
-   "display_name": "pytorch3d (local)",
+   "display_name": "Python 3",
    "language": "python",
-   "name": "pytorch3d_local"
+   "name": "python3"
   },
   "language_info": {
    "codemirror_mode": {
@@ -687,7 +687,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.7.5+"
+   "version": "3.7.6"
   },
   "widgets": {
    "application/vnd.jupyter.widget-state+json": {