Point clouds to volumes

Summary:
Conversion from point clouds to volumes

```
Benchmark                                                        Avg Time(μs)      Peak Time(μs) Iterations
--------------------------------------------------------------------------------
ADD_POINTS_TO_VOLUMES_10_trilinear_[25, 25, 25]_1000                 43219           44067             12
ADD_POINTS_TO_VOLUMES_10_trilinear_[25, 25, 25]_10000                43274           45313             12
ADD_POINTS_TO_VOLUMES_10_trilinear_[25, 25, 25]_100000               46281           47100             11
ADD_POINTS_TO_VOLUMES_10_trilinear_[101, 111, 121]_1000              51224           51912             10
ADD_POINTS_TO_VOLUMES_10_trilinear_[101, 111, 121]_10000             52092           54487             10
ADD_POINTS_TO_VOLUMES_10_trilinear_[101, 111, 121]_100000            59262           60514              9
ADD_POINTS_TO_VOLUMES_10_nearest_[25, 25, 25]_1000                   15998           17237             32
ADD_POINTS_TO_VOLUMES_10_nearest_[25, 25, 25]_10000                  15964           16994             32
ADD_POINTS_TO_VOLUMES_10_nearest_[25, 25, 25]_100000                 16881           19286             30
ADD_POINTS_TO_VOLUMES_10_nearest_[101, 111, 121]_1000                19150           25277             27
ADD_POINTS_TO_VOLUMES_10_nearest_[101, 111, 121]_10000               18746           19999             27
ADD_POINTS_TO_VOLUMES_10_nearest_[101, 111, 121]_100000              22321           24568             23
ADD_POINTS_TO_VOLUMES_100_trilinear_[25, 25, 25]_1000                49693           50288             11
ADD_POINTS_TO_VOLUMES_100_trilinear_[25, 25, 25]_10000               51429           52449             10
ADD_POINTS_TO_VOLUMES_100_trilinear_[25, 25, 25]_100000             237076          237377              3
ADD_POINTS_TO_VOLUMES_100_trilinear_[101, 111, 121]_1000             81875           82597              7
ADD_POINTS_TO_VOLUMES_100_trilinear_[101, 111, 121]_10000           106671          107045              5
ADD_POINTS_TO_VOLUMES_100_trilinear_[101, 111, 121]_100000          483740          484607              2
ADD_POINTS_TO_VOLUMES_100_nearest_[25, 25, 25]_1000                  16667           18143             31
ADD_POINTS_TO_VOLUMES_100_nearest_[25, 25, 25]_10000                 17682           18922             29
ADD_POINTS_TO_VOLUMES_100_nearest_[25, 25, 25]_100000                65463           67116              8
ADD_POINTS_TO_VOLUMES_100_nearest_[101, 111, 121]_1000               48058           48826             11
ADD_POINTS_TO_VOLUMES_100_nearest_[101, 111, 121]_10000              53529           53998             10
ADD_POINTS_TO_VOLUMES_100_nearest_[101, 111, 121]_100000            123684          123901              5
--------------------------------------------------------------------------------
```

Output with `DEBUG=True`
{F338561209}

Reviewed By: nikhilaravi

Differential Revision: D22017500

fbshipit-source-id: ed3e8ed13940c593841d93211623dd533974012f

pytorch3d/ops/__init__.py

@@ -14,6 +14,10 @@
     estimate_pointcloud_local_coord_frames,
     estimate_pointcloud_normals,
 )
+from .points_to_volumes import (
+    add_pointclouds_to_volumes,
+    add_points_features_to_volume_densities_features,
+)
 from .sample_points_from_meshes import sample_points_from_meshes
 from .subdivide_meshes import SubdivideMeshes
 from .utils import (