Make positional args optional

[ericwtodd]

When using the NNsight class to wrap pre-existing pytorch modules (e.g. CLIP-ViT) where positional arguments are optional, users are currently forced to provide a positional argument (e.g. inputs).

Positional arguments to the NNsight forward should be made optional when the underlying model allows for it.

See discord discussion about this issue for a CLIP-VIT model: Discussion link