[export] allow user input mutation in aot_export (#119356)
This PR enables input mutation in aot_export by removing the guard and ensuring that the GraphSignature is properly wired up.
This allows to undo the gross hack in torch.export where we lift user inputs to buffers in order to get around aot_export upstream support. It also makes input mutation work properly for non-strict mode.
Mutations on inputs that require_grad are still banned (I added a test for a non-parameter input as well, just to make sure).
Differential Revision: [D53507440](https://our.internmc.facebook.com/intern/diff/D53507440/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119356
Approved by: https://github.com/bdhirsh, https://github.com/zhxchen17, https://github.com/titaiwangms
