Fix executorch kv cache incompatibility with to_executorch lowering

[dvorjackz]

Summary

Fix the Llama 3.2 vision text decoder prefill issue by marking the kv cache as an initialized mutable buffer in a custom pass

Test plan

• Add kv_cache export tests that test accuracy against torchtune eager and verify contents of the cache after prefill and token-by-token generation
• Export and run full Llama 3.2 vision text decoder

> 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.pte" -d fp32 --verbose --max_seq_length 64 -k
> python -m examples.models.llama3_2_vision.runner.native --model llama3_2_vision --pte llama3_2_vision.pte  --tokenizer /tmp/Llama-3.2-11B-Vision-Instruct/original/tokenizer.model --prompt "Who's the founder of Meta?" --params examples/models/llama3_2_vision/text_decoder/params/demo_config.json --max_len 64 -kv --temperature 0

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/7279

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❌ 1 New Failure

As of commit e297c9b with merge base 343aa0c ():

NEW FAILURE - The following job has failed:

• pull / unittest-arm / linux-job (gh)
  RuntimeError: Command docker exec -t b90672cd8630d1544e16c7c60d45fd7508caeb7cb8b1bc44652f5f7c7e8f80fe /exec failed with exit code 1

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[tarun292]

We should raise an exception here if we don't find the buffer.