Avoid pickling unstable reference to moved proto classes. (#23739)

CloudPickle notices that schema_pb2 (from the closure of the locally
defined __reduce__ method) is not importable under its declared name
(org.apache.beam...) and tries to pickle it (which fails due to proto
classes themselves being unpicklable).  This avoids that error by
moving it out.

sdks/python/apache_beam/typehints/schemas.py

@@ -463,19 +463,24 @@ def named_tuple_from_schema(self, schema: schema_pb2.Schema) -> type:
 
     # Define a reduce function, otherwise these types can't be pickled
     # (See BEAM-9574)
-    def __reduce__(self):
-      return (
-          _hydrate_namedtuple_instance,
-          (schema.SerializeToString(), tuple(self)))
-
-    setattr(user_type, '__reduce__', __reduce__)
+    setattr(
+        user_type,
+        '__reduce__',
+        _named_tuple_reduce_method(schema.SerializeToString()))
 
     self.schema_registry.add(user_type, schema)
     coders.registry.register_coder(user_type, coders.RowCoder)
 
     return user_type
 
 
+def _named_tuple_reduce_method(serialized_schema):
+  def __reduce__(self):
+    return _hydrate_namedtuple_instance, (serialized_schema, tuple(self))
+
+  return __reduce__
+
+
 def _hydrate_namedtuple_instance(encoded_schema, values):
   return named_tuple_from_schema(
       proto_utils.parse_Bytes(encoded_schema, schema_pb2.Schema))(*values)