tree_flatten_spec is bad; it isn't synced up with `register_pytree_node` so it will not handle arbitrary custom pytrees. It's also not really maintained.
We only use it for two purposes:
- To retain kwarg ordering stability, so that if the user passes in kwargs in a different order things will still work.
- To do "structural" checks that ignore types.
In both cases, tree_flatten_spec is probably *not* the ideal way to implement the desired behavior.
## kwargs ordering
- tree_flatten_spec overwrites the behavior of ALL dictionaries, not just kwargs. This is not correct, dictionary ordering is meaningful in Python, and it's pretty trivial to write a program that relies on dict ordering.
- For kwargs, we do sort of expect that the order in which arguments are passed shouldn't matter. BUT there is one exception: `**kwargs`. In fact, [PEP 468](https://peps.python.org/pep-0468/) was introduced specifically to clarify that ordering does matter when the function being called uses `**kwargs`.
In this diff I introduce a utility function that *only* reorders kwargs. This gets us most of the way to correct—dicts are no longer reordered, but kwargs can be passed in any order.
A "fully correct" solution would need fix the corner case from PEP468. We could detect whether the top-level fn being traced uses `**kwargs` (via `inspect`), then serialize a flag for it. In ExportedProgram, we would check that flag and only re-order if `**kwargs` was unused; otherwise error if the key order doesn't match. This is a super corner case though, so I'll file it as a followup task.
## structural equivalence checking
This is another use case, where again `tree_flatten_spec` is too broad. Generally we want to treat a precise two types as the same, not override the behavior of comparison generally. So I introduce an `is_equivalent` util for this purpose.
Differential Revision: [D53168420](https://our.internmc.facebook.com/intern/diff/D53168420/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/118608
Approved by: https://github.com/zhxchen17
ghstack dependencies: #118607
