Register dataclass at definition so that unpickling the dataclass object, type, and treedef works in most cases (with one exception).

chex/_src/dataclass.py

@@ -214,15 +214,31 @@ def _replace(self, **kwargs):
  def _getstate(self):
  return self.__dict__
 
- # Patch __setstate__ to register the object on deserialization.
+ # Register the dataclass at definition. As long as the dataclass is defined
+ # outside __main__, this is sufficient to make JAX's PyTree registry
+ # recognize the dataclass and the dataclass' custom PyTreeDef, especially
+ # when unpickling either the dataclass object, its type, or its PyTreeDef,
+ # in a different process, because the defining module will be imported.
+ #
+ # However, if the dataclass is defined in __main__, unpickling in a
+ # subprocess does not trigger re-registration. Therefore we also need to
+ # register when deserializing the object, or construction (e.g. when the
+ # dataclass type is being unpickled). Unfortunately, there is not yet a way
+ # to trigger re-registration when the treedef is unpickled as that's handled
+ # by JAX.
+ #
+ # See internal dataclass_test for unit tests demonstrating the problems.
+ register_dataclass_type_with_jax_tree_util(dcls)
+
+ # Patch __setstate__ to register the dataclass on deserialization.
  def _setstate(self, state):
  register_dataclass_type_with_jax_tree_util(dcls)
  self.__dict__.update(state)
 
  orig_init = dcls.__init__
 
- # Patch object's __init__ such that the class is registered on creation if
- # it is not registered on deserialization.
+ # Patch __init__ such that the dataclass is registered on creation if it is
+ # not registered on deserialization.
  @functools.wraps(orig_init)
  def _init(self, *args, **kwargs):
  register_dataclass_type_with_jax_tree_util(dcls)