[llama-mm] Make text decoder exportable

[larryliu0820]

Summary: Adds source transformation and changes example input to make text decoder exportable.

Test Plan: Added new unit test.

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Summary

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Test plan

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:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/6999

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As of commit c6909a74bb8862312c11db7140036c7cda3b5824 with merge base 6ac19cc2d69e34d30668fcd9da9ea5554a6af576 (): :green_heart: Looks good so far! There are no failures yet. :green_heart:

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