Add PythonState

stubs/tensorflow/tensorflow/train/experimental.pyi

@@ -1,3 +1,13 @@
+import abc
 from _typeshed import Incomplete
+from typing_extensions import Self
+
+from tensorflow.python.trackable.base import Trackable
+
+class PythonState(Trackable, metaclass=abc.ABCMeta):
+    @abc.abstractmethod
+    def serialize(self) -> str: ...
+    @abc.abstractmethod
+    def deserialize(self, string_value: str) -> Self: ...
 
 def __getattr__(name: str) -> Incomplete: ...