[executorch][docs] Don't generate docs for exir.serialize

This module is now hidden, so we shouldn't create docs for it. Also its new name is `exir._serialize`, so the current names cause the docs build to break.

Differential Revision: [D49331927](https://our.internmc.facebook.com/intern/diff/D49331927/)

ghstack-source-id: 200910882
Pull Request resolved: #371

Author: dbort

docs/source_py/exir.rst

@@ -120,4 +120,3 @@ Subpackages
 
    exir.data_structures
    exir.passes
-   exir.serialize