Scale leaves verts normals unchanged

Summary:
There is no need to recalculate normals when scaling a mesh by a constant. We omit doing this for vertex normals. This will make things less confusing when we allow user-specified vertex normals.

Face normals also don't need recalculating, but they are calculated at the same time as face areas which do, so it is easiest not to change their behavior here. That is convenient because we are not immediately planning to allow user-specified face normals.

Reviewed By: nikhilaravi

Differential Revision: D27793476

fbshipit-source-id: 827f1be4bc78bf0391ce3959cce48c4f3ee326fe

pytorch3d/structures/meshes.py

@@ -1336,15 +1336,13 @@ def scale_verts_(self, scale):
                 if len(verts) > 0:
                     self._verts_padded[i, : verts.shape[0], :] = verts
 
-        # update face areas and normals and vertex normals
+        # update face areas and normals
         # only if the original attributes are computed
         if any(
             v is not None
             for v in [self._faces_areas_packed, self._faces_normals_packed]
         ):
             self._compute_face_areas_normals(refresh=True)
-        if self._verts_normals_packed is not None:
-            self._compute_vertex_normals(refresh=True)
         return self
 
     def scale_verts(self, scale):