Codemod examples/models/llama2 to examples/models/llama (#6302)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/6302

Pull Request resolved: https://github.com/pytorch/executorch/pull/6275

Squash of D64446460, D64446461, D64446462, D64446459

allow-large-files
bypass-github-export-checks
bypass-github-pytorch-ci-checks
bypass-github-executorch-ci-checks
bypass-github-torchtune-ci-checks

Reviewed By: iseeyuan

Differential Revision: D64446900

fbshipit-source-id: f3cfe3d049470581ca1636042fd89f1c4fac2726