[llama-mm] Add export-friendly tile position embedding (#6671)

Summary:

Before we make a decision on whether torchtune takes this
export-friendly version of `TilePositionEmbedding`, we put it under
`extension/llm` so that users can start to use it.

Added unit tests to make sure the behavior is the same as the reference
implementation in torchtune and export/AOTI/ET all working properly.

Test Plan:

Reviewers:

Subscribers:

Tasks:

Tags:

ghstack-source-id: fe65ec6c590b6579ac68847cd2e3d4a09921b7e5
Pull Request resolved: https://github.com/pytorch/executorch/pull/6650

Co-authored-by: Mengwei Liu <larryliu@meta.com>