ppx_op: Include config.label in the label of the primary (returned) tensor, when available, and not incorporating a label from a function argument

[lukstafi]

That's as informative as including it in the inline-declared tensors.

[lukstafi]

This feature might interact badly with #210 leading to labels that are too bloated. It would still make sense if there is no function (tensor) argument, but such cases would maybe be rare. So with this new consideration it's low priority.