Fix user backend flag bad assignment

[LvisRoot]

Fixes user_requested_backend bad assignment when backend is not specified

• Changed argument backend to backend_name
• Check backend_name to set user_requested_backend instead of the created backend object

[Caenorst]

Hi @LvisRoot , what are you trying to resolve?

I can see some backend_name in the code base but all the constructors use backend:

• from_camera_pose: https://github.com/NVIDIAGameWorks/kaolin/blob/master/kaolin/render/camera/extrinsics.py#L278
• from_lookat: https://github.com/NVIDIAGameWorks/kaolin/blob/master/kaolin/render/camera/extrinsics.py#L331
• from_view_matrix: https://github.com/NVIDIAGameWorks/kaolin/blob/master/kaolin/render/camera/extrinsics.py#L417

Since it's not inconsistent (I did find an inconsistency here https://github.com/NVIDIAGameWorks/kaolin/blob/master/kaolin/render/camera/extrinsics.py#L245), and is not causing any known issue I would suggest to avoid breaking change.

[LvisRoot]

Hi @Caenorst , thanks for the remark, here's what I'm trying to fix:

The issue is that in from_view_matrix: https://github.com/NVIDIAGameWorks/kaolin/blob/a1b2ad427a580c10f4a3667c31565f244a911aaf/kaolin/render/camera/extrinsics.py#L474 both the passed backend argument (which is a backend name as a string) has the same variable name as the backend instanciated in the function.

So extrinsics._shared_fields['user_requested_backend'] gets always set, since you just constructed a backend object and is always not None, regardless of a backend name being passed to the function or not. This breaks auto backend selection for optimization when you run, say camera.requires_grad_(True) if you constructed the extrinsics with from_view_matrix.

I see changing the argument name would break other stuff, so I'll create an aux variable to store the backend instance, keep the backend argument name.

would you do it differently?