Motivation

A "structseq" is this weird Python-C API thing used to create namedtuple-like types from C++.

#75915 introduces some logic that ideally would grab all "structseq" classes from torch.return_types and register them as PyTree nodes. However, we didn't know how to actually check if something is a structseq or not (the documentation doesn't provide any helper functions for that).

If possible we should write an API that can check if a type is a "structseq" and then use it in #75915