Bug in obj_io.py with shapenet

[naifuliang]

🐛 Bugs / Unexpected behaviors

Error Information:

line 635, in _load_obj
    face_material_names = np.array(material_names)[idx]  # (F,)
IndexError: index -1 is out of bounds for axis 0 with size 0

Instructions To Reproduce the Issue:

https://github.com/facebookresearch/pytorch3d/blob/main/pytorch3d/datasets/shapenet/shapenet_core.py line

        # 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] = ""

If idx ==-1, np.array(material_names)[-1] will be executed, which causes an error.

[bottler]

If this was still an issue then 9acdd67 must have squashed it just now.