Fix exir deserialization logic (#2857)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2857

Looks like a recent str -> TensorArgument change in diff D54855251 wasn't very thorough. ETRecord deserialization failed because of it.

Reviewed By: Jack-Khuu, angelayi

Differential Revision: D55776877

fbshipit-source-id: 2e0428a1c0cd0e7e2e8d7a838ba611020b5716a1