fix obj material indexing bug #1368

Summary:
Make the negative index actually not an error

fixes #1368

Reviewed By: das-intensity

Differential Revision: D62177991

fbshipit-source-id: e5ed433bde1f54251c4d4b6db073c029cbe87343

Author: bottler

pytorch3d/io/obj_io.py

@@ -649,8 +649,7 @@ def _load_obj(
         # Create an array of strings of material names for each face.
         # If faces_materials_idx == -1 then that face doesn't have a material.
         idx = faces_materials_idx.cpu().numpy()
-        face_material_names = np.array(material_names)[idx]  # (F,)
-        face_material_names[idx == -1] = ""
+        face_material_names = np.array([""] + material_names)[idx + 1]  # (F,)
 
         # Construct the atlas.
         texture_atlas = make_mesh_texture_atlas(