Make more functions/modules/traits public

[M1ngXU]

When creating new models I always find myself needing to use "private" functions/modules/traits, and I thus have my own branch of dfdx that exposes more "internal" stuff, especially:

• Gradients: It's impossible to implement a custom Model with its own backpropagation as all methods are pub(crate)
• TensorLike/GhostTensor: TensorLike is useful to get a reference to the data without cloning it and GhostTensor for Gradients (though it's not necessary)
• special cpu/cuda functions: index_to_i, launch_cfg are also required to compute forward/backward passes
• and probably much more ...

Can the visibility be changed, maybe only with an additional feature?

[coreylowman]

Yeah just open PRs for these honestly, we can discuss more specifics on each one individually!

[coreylowman]

Mentioning #814 which also made a variety of apis public

[coreylowman]

I'm going to close this for now - if anyone has specific requests, just open a PR with the change, definitely open to whatever people want (I've discovered a lot through trying to make an external nn layer)