support static fields in treemath.struct

[GallagherCommaJack]

it would be nice to have some fields be pytree nodes and others not, which would make this a full replacement for flax.struct

[shoyer]

Yes, this would be nice. I would happily accept a PR adding this feature.

[ricor07]

Hello, I'd like to work on the project by fixing this issue. Can you assign this to me? Thanks

[shoyer]

Feel free to send in a PR!

[ricor07]

Good evening. Just for clarification, I don't understand well what the issue wants. I thought of making pytree_values and non_pytree_values arrays in the tree_flatten and tree_unflatten function. Am I right or am I missing something? Thank you