[FX] Add a default_value arg to Graph.placeholder and fix split_module (#71016)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71016
I found out that `split_module` doesn't preserve default values for arguments. In trying to fix that, I noticed that `Graph.placeholder` doesn't make it easy to add a default argument when making a placeholder. This PR addresses both of those issues
Test Plan: Imported from OSS
Reviewed By: ansley
Differential Revision: D33482218
Pulled By: jamesr66a
fbshipit-source-id: 57ebcdab25d267333fb1034994e08fc1bdb128ee
