Ability to specify full file configs for export_llm

[jackzhxng]

Hydra is a bit opinionated on how config files should be provided to the CLI, this adds some tooling so that we can specify a config file from any arbitrary location to use with export_llm.

Example:

python -m extension.llm.export.export_llm --config ...some-path.../example_llm_config.yaml

[jackzhxng]

Stack from ghstack (oldest at bottom):

• Use export_llm in CI #11836
• Update export_llama in READMEs to use export_llm #11811
• Fix LlmConfig enum usage #11833
• -> Ability to specify full file configs for export_llm #11809

[pytorch-bot]

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[github-actions]

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[mergennachin]

As part of the 0.7, one deliverable would be to swap the ci with the new API. Not in this PR though

executorch/.ci/scripts/test_llama.sh

Line 254 in 496cb05

$PYTHON_EXECUTABLE -m examples.models.llama.export_llama ${EXPORT_ARGS}

[mergennachin]

I think we should just move these to https://github.com/pytorch/executorch/blob/main/requirements-dev.txt

[mergennachin]

Possibly no need