Change visibility of nn::Path::add method to public.

[antoniprzybylik]

Description

This pull request updates the visibility of the nn::Path::add method from pub(crate) to pub. The updated method signature is:

pub fn add(&self, name: &str, tensor: Tensor, trainable: bool) -> Tensor

Reason for Change

Previously, the add method was restricted to crate-level visibility (pub(crate)), which limited its usage. This change makes the method accessible outside the crate, enabling users to add existing tensors to a VarStore.

Use Case

While implementing Python bindings, I needed to add tensors from Python to VarStore. Making the add method public makes this possible.