Add exported program runner

[dvorjackz]

Summary

Add an ExportedProgram runner for TorchTune Llama.

Test plan

# Download resources
tune download meta-llama/Llama-3.2-11B-Vision-Instruct --output-dir /tmp/Llama-3.2-11B-Vision-Instruct

# Export model
python -m examples.models.llama.export_llama --model llama3_2_vision --checkpoint /tmp/Llama-3.2-11B-Vision-Instruct/original/consolidated.pth --params examples/models/llama3_2_vision/text_decoder/params/demo_config.json  --metadata '{"append_eos_to_prompt": 0, "get_bos_id":128000, "get_eos_ids":[128009, 128001], "get_n_bos": 0, "get_n_eos": 0}' --output_name="llama3_2_vision.pt2" -d fp32 --verbose --max_seq_length 64 --export_only -kv

# Run ExportedProgram
python -m examples.models.llama3_2_vision.runner.exported --model llama3_2_vision --pt2 llama3_2_vision.pt2  --tokenizer /tmp/Llama-3.2-11B-Vision-Instruct/original/tokenizer.model --prompt "How many calories are in bread?" --params examples/models/llama3_2_vision/text_decoder/params/demo_config.json --max_seq_length 64 -kv

Output:

The number of calories in bread can vary greatly depending on the type of bread, its ingredients, and its size. Here are the approximate calorie counts for different types of bread:                                                                                                        
White bread: 80-100 calories per slice

[pytorch-bot[bot]]

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/6969

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As of commit d595735f345564da55b04c92a0e35bc13648eb8a with merge base 711f1c23d32acd798dabf0028b31e8fcb2c2487f (): :green_heart: Looks good so far! There are no failures yet. :green_heart:

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This pull request was exported from Phabricator. Differential Revision: D66186052