Bugfix - normalize emitted ray directions after substraction

Summary: The bug lead to non-coinciding origins of the rays emitted from perspective cameras when unit_directions=True

Reviewed By: bottler

Differential Revision: D40865610

fbshipit-source-id: 398598e9e919b53e6bea179f0400e735bbb5b625

Author: davnov134

pytorch3d/renderer/implicit/raysampling.py

@@ -670,12 +670,13 @@ def _xy_to_ray_bundle(
 
     # directions are the differences between the two planes of points
     rays_directions_world = rays_plane_2_world - rays_plane_1_world
-    if unit_directions:
-        rays_directions_world = F.normalize(rays_directions_world, dim=-1)
 
     # origins are given by subtracting the ray directions from the first plane
     rays_origins_world = rays_plane_1_world - rays_directions_world
 
+    if unit_directions:
+        rays_directions_world = F.normalize(rays_directions_world, dim=-1)
+
     return RayBundle(
         rays_origins_world.view(batch_size, *spatial_size, 3),
         rays_directions_world.view(batch_size, *spatial_size, 3),