[fx][acc_tracer] fix defaulted placeholder normalization (#73406)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73406
Placeholder defaults are stored in `node.args`, during normalization we had dropped these. This diff passes the default args through the normalization transformation.
Test Plan:
Added tests to cover cases with optional inputs, test covers
* nothing passed to optional input
* `None` passed to optional input
* a tensor passed to optional input
Reviewed By: jfix71
Differential Revision: D34463493
fbshipit-source-id: f0c3a4083cb3dd4a69111a758561f0d2c0609787
(cherry picked from commit 7fb482cbfc34077426efa18ac74311bd4533dcdf)
