Use ExternalProject_Add to add extension/llm/tokenizers library

[kirklandsign]

🚀 The feature, motivation and pitch

Right now we are using add_subdirectory like this:

executorch/examples/models/llama/runner/CMakeLists.txt

Lines 68 to 71 in 24789c8

	add_subdirectory(
	${EXECUTORCH_ROOT}/extension/llm/tokenizers
	${CMAKE_CURRENT_BINARY_DIR}/tokenizers
	)

One issue is sub-libraries doesn't have namespace, so if any deps has same name, we get conflict and error. For example, #6767 is blocked.

Let's see if ExternalProject_Add could help.

Alternatives

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Additional context

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RFC (Optional)

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cc @larryliu0820 @jathu