[Llava] Add max_context_len CLI arg

[pytorchbot]

Summary

Add a required max_context_len argument to the Llava example model export. When set to 768, this reduces the memory consumption (~6GiB -> ~4.8GiB RSS) at the cost of a smaller context length and thus fixes #14474.

Test plan

Ran ./test_llava.sh and validated the reported memory consumption on an x86 Linux machine.

I 00:00:18.433471 executorch:main.cpp:172] Starting generation...
I 00:00:18.433500 executorch:multimodal_runner.cpp:95] RSS after loading model: 4746.726562 MiB (0 if unsupported)
I 00:00:18.433554 executorch:multimodal_runner.cpp:119] Prefilling input 0/3, type: text
I 00:00:19.484581 executorch:multimodal_runner.cpp:119] Prefilling input 1/3, type: image
I 00:00:19.484710 executorch:multimodal_prefiller.cpp:83] Image tensor dim: 3, dtype: Byte
I 00:00:30.442685 executorch:multimodal_runner.cpp:119] Prefilling input 2/3, type: text
I 00:00:30.951938 executorch:multimodal_runner.cpp:138] RSS after multimodal input processing: 4847.933594 MiB (0 if unsupported)
I 00:00:30.952000 executorch:multimodal_runner.cpp:148] Max new tokens resolved: 153, pos_ 615, max_context_len 768

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15112

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❌ 1 New Failure, 4 Cancelled Jobs

As of commit d0bf6bb with merge base e0dda90 ():

NEW FAILURE - The following job has failed:

• Lint / lintrunner-mypy / linux-job (gh)
  >>> Lint for docs/source/conf.py:

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• pull / test-llava-runner-linux / linux-job (gh)
  ##[error]The operation was canceled.
• pull / unittest / macos / macos-job (gh)
  ##[error]The operation was canceled.
• pull / unittest-editable / macos / macos-job (gh)
  ##[error]The operation was canceled.
• trunk / unittest-release / macos / macos-job (gh)
  ##[error]The operation was canceled.

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