[llama-mm] Add export-friendly tile position embedding

[larryliu0820]

Stack from ghstack (oldest at bottom):

• [llama-mm] Onboard Llama3.2 mm vision encoder #6653
• -> [llama-mm] Add export-friendly tile position embedding #6650

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:

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/6650

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❌ 2 New Failures

As of commit b0d9e3f with merge base 3a1f8d2 ():

NEW FAILURES - The following jobs have failed:

• pull / test-binary-size-linux-gcc / linux-job (gh)
  RuntimeError: Command docker exec -t 07643381d18b7b36f281f84e6f7e6161f75cb7f377a1736b4324db8b546d897c /exec failed with exit code 1
• pull / unittest-arm / linux-job (gh)
  RuntimeError: Command docker exec -t 1a41ab59b6e1a95189b4c1cf4adef8f810bd5fd8258b780d4a2742efdcff26bd /exec failed with exit code 1

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