Example LLM skeleton app (#26)

Author: mergennachin

llm/README.md

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+This is a WIP directory to migrate https://github.com/pytorch/executorch/tree/main/examples/demo-apps

llm/android/README.md

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+Example Android app that uses LLM Java/Kotlin APIs in extension/llm. Priority P0

llm/apple/ios/README.md

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+Example iOS app that uses LLM Swift APIs in extension/llm. Priority P0

llm/cpp/README.md

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+Example C++ native runner that uses C++ LLM runtime API. Priority P1

llm/python/README.md

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+# Setup
+
+# Export a model
+
+## Export a model using export_llm script
+
+## Export a model using HuggingFace optimum