[pytree] add APIs to determine a class is a namedtuple or PyStructSequence (#113257)

Changes in this PR:

1. Add `is_structseq` and `is_structseq_class` functions to determine a object or a class is PyStructSequence.
2. Add a generic class `structseq` which can be used as the registration key for PyStructSequence types like `namedtuple` for Named Tuple types.
3. Change `is_namedtuple` to accept subclasses of namedtuple to be namedtuple. Before this PR, only namedtuple class directly created by `collections.namedtuple` or `typing.NamedTuple` were namedtuple classes while their subclasses were not. This PR makes `is_namedtuple` return true for subclasses of namedtuple class.

Resolves #75982. New tests are included in this PR.

- #75982

Pull Request resolved: https://github.com/pytorch/pytorch/pull/113257
Approved by: https://github.com/zou3519

torch/utils/_cxx_pytree.py

@@ -23,7 +23,15 @@ import optree
 from optree import PyTreeSpec as TreeSpec  # direct import for type annotations
 
 import torch.utils._pytree as python_pytree
-from torch.utils._pytree import KeyEntry as KeyEntry
+from torch.utils._pytree import (
+    is_namedtuple as is_namedtuple,
+    is_namedtuple_class as is_namedtuple_class,
+    is_namedtuple_instance as is_namedtuple_instance,
+    is_structseq as is_structseq,
+    is_structseq_class as is_structseq_class,
+    is_structseq_instance as is_structseq_instance,
+    KeyEntry as KeyEntry,
+)
 
 
 __all__ = [
@@ -39,6 +47,7 @@ __all__ = [
     "keystr",
     "key_get",
     "register_pytree_node",
+    "tree_is_leaf",
     "tree_flatten",
     "tree_flatten_with_path",
     "tree_unflatten",
@@ -58,6 +67,12 @@ __all__ = [
     "treespec_dumps",
     "treespec_loads",
     "treespec_pprint",
+    "is_namedtuple",
+    "is_namedtuple_class",
+    "is_namedtuple_instance",
+    "is_structseq",
+    "is_structseq_class",
+    "is_structseq_instance",
 ]