[CMake] Potentially duplicated srcs in llama_runner build

🐛 Describe the bug

For example

set(_llama_runner__srcs
    examples/models/llama/runner/runner.cpp
    examples/models/llama/tokenizer/llama_tiktoken.cpp
    extension/evalue_util/print_evalue.cpp
    extension/llm/runner/text_decoder_runner.cpp
    extension/llm/runner/text_prefiller.cpp
    extension/llm/sampler/sampler.cpp <--------------------
    extension/llm/tokenizers/src/bpe_tokenizer_base.cpp
    extension/llm/tokenizers/src/hf_tokenizer.cpp
    extension/llm/tokenizers/src/llama2c_tokenizer.cpp
    extension/llm/tokenizers/src/pre_tokenizer.cpp
    extension/llm/tokenizers/src/re2_regex.cpp
    extension/llm/tokenizers/src/regex.cpp
    extension/llm/tokenizers/src/tiktoken.cpp
    extension/llm/tokenizers/src/token_decoder.cpp
    extension/llm/tokenizers/third-party/llama.cpp-unicode/src/unicode-data.cpp
    extension/llm/tokenizers/third-party/llama.cpp-unicode/src/unicode.cpp
    extension/tensor/tensor_ptr.cpp
    extension/tensor/tensor_ptr_maker.cpp
)

set(_extension_llm_runner__srcs
    extension/data_loader/file_data_loader.cpp
    extension/data_loader/mmap_data_loader.cpp
    extension/llm/runner/text_decoder_runner.cpp
    extension/llm/runner/text_prefiller.cpp
    extension/llm/sampler/sampler.cpp <------------ again!
    extension/tensor/tensor_ptr.cpp
    extension/tensor/tensor_ptr_maker.cpp
)

and even

set(_llava_runner__srcs
    "${CMAKE_CURRENT_SOURCE_DIR}/llava_runner.cpp"
    "${EXECUTORCH_ROOT}/extension/llm/sampler/sampler.cpp" <-----
    "${EXECUTORCH_ROOT}/extension/llm/tokenizers/src/llama2c_tokenizer.cpp"
)

Note that we might build llava and llama runner into the same target, for example here
https://github.com/pytorch/executorch/blob/306430825c2ffe2061221a197904e2f79e628998/extension/android/CMakeLists.txt#L137-L141

Versions

main 306430825c2ffe2061221a197904e2f79e628998

cc @larryliu0820 @jathu