Add lift constant tensors passes after aten_to_edge (#359)

Summary:
X-link: https://github.com/pytorch/pytorch/pull/109382

Pull Request resolved: https://github.com/pytorch/executorch/pull/359

When exporting using enable_aot (through the torch.export path), we want to lift all constant tensors as buffers to the exported program. The ScalarToTensor pass in EXIR's aten_to_edge passes will create some constant tensors in the graph, so we will need to run a lift_constant_tensors pass afterwards.

Note that this only needs to be applied when exporting using the torch.export path because in the original path, nothing is lifted.

Reviewed By: cccclai

Differential Revision: D49207492

fbshipit-source-id: 971c44d109b41de8b3f9ff0565fd39bdd3a17b9f