Don't generate docs for exir.serialize (#371)

Summary:
Pull Request resolved: #371

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.
ghstack-source-id: 200910882
exported-using-ghexport

Reviewed By: svekars

Differential Revision: D49331927

fbshipit-source-id: 478dbfd47f1177454f5242cefc864394f298bc27

Author: dbort

docs/source_py/exir.rst

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