Parameter structure abstraction

[Ebanflo42]

This might be a trivial one, but it's important from a usability perspective.

PjRt executables expect a slice of buffers or literals as input to run on. However, if you have a model with dozens of parameter tensors organizing them all into a slice manually becomes tedious (already visible in the mnist_xla example), so I think our abstract model API should basically be callable on any user-defined structure implementing Into<Vec<Literal>> or Into<Vec<PjRtBuffer>>. The gradient engine should also return gradients with the same desired structure if the parameter struct implements From<Vec<T>>.

This is basically the equivalent of JAX tree flattening/unflattening

I don't think we have to do anything very thoughtful for this one, just write the type abstract signatures and call into and from appropriately.

[Ebanflo42]

@atlv24 could you add the definition of your typeclass and its macros to a file called src/tree.rs