Update examples/models/llama/README.md to fix build broken

[DamonFool]

Fix the following error

CMake Error at tools/cmake/common/preset.cmake:122 (message):
  Use of 'EXECUTORCH_BUILD_EXTENSION_MODULE' requires
  'EXECUTORCH_BUILD_EXTENSION_FLAT_TENSOR'
Call Stack (most recent call first):
  tools/cmake/preset/default.cmake:300 (check_required_options_on)
  CMakeLists.txt:87 (include)

[pytorch-bot]

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[lucylq]

Hi @DamonFool, thanks for submitting this PR. I don't think llama build should require flat_tensor - is the repro command exactly from the readme/could you provide instructions for your repro?

[DamonFool]

Hi @DamonFool, thanks for submitting this PR. I don't think llama build should require flat_tensor - is the repro command exactly from the readme/could you provide instructions for your repro?

Thanks @lucylq for taking a look at this.
Yes, I just followed the steps in the readme and copied commands in it.

The logic is that llama requires EXECUTORCH_BUILD_EXTENSION_MODULE, which requires EXECUTORCH_BUILD_EXTENSION_FLAT_TENSOR.

The cmake error msg has showed that dependency clearly.
You can verify it by testing the commands it the readme.
Thanks.