Part of #109684
- #109684
Changes:
- Add new functions `tree_structure`, `tree_leaves`, `tree_map_` and `tree_map_only_` to Python pytree.
- Extract reusable tests for pytree to `TestGenericPytree`.
- Change `treespec_dumps` and `treespec_loads` in C++ pytree to call Python pytree and use JSON string as serialization type.
- Rename `torch.utils.pytree` -> `torch.utils._cxx_pytree`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/110395
Approved by: https://github.com/zou3519
