[export] Allow optional call-spec (#105041)
Summary: Submodules may have a none call-spec values, which is ok. Updating types + serializer to handle this
Test Plan: CI
Differential Revision: D47353101
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105041
Approved by: https://github.com/ydwu4, https://github.com/zhxchen17
