Refactor CMake build for examples/models/llama

[GregoryComer]

Summary

Update the CMake build for examples/models/llama to be in-tree in the top-level build but excluded from all. This means that it can be built in a single step, but will only be built when explicitly requested with --target llama_main.

This simplifies the build quite a bit (as evidenced by how many lines we remove in this PR) and prevents issues with iterative rebuild or mismatched build options between llama_main and the core runtime.

Note that the top-level make llama-cpu command is unmodified, just simplified under the hood. For direct CMake builds, here's a before and after:

Before:

cmake --preset llm -DCMAKE_INSTALL_PREFIX=cmake-out
cmake --build cmake-out --target install
cd examples/models/llama
cmake -B ../../../cmake-out/examples/models/llama -DCMAKE_FIND_ROOT_PATH=../../../cmake-out
cmake --build ../../../cmake-out/examples/models/llama --target llama_main

After:

cmake -B cmake-out --preset llm
cmake --build cmake-out --target llama_main

[pytorch-bot]

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• Rolling out OSDC (ARC) runners on pull workflow for PyTorch trunk commits

❌ 16 New Failures, 2 Cancelled Jobs, 2 Pending, 3 Unrelated Failures

As of commit 965a4a9 with merge base 74403e2 ():

NEW FAILURES - The following jobs have failed:

• MLX / test-mlx-llm (unsloth/gemma-3-1b-it, gemma3-1b, false, 4w) / test-mlx-llm-gemma3-1b-4w (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 2
• MLX / test-mlx-llm (unsloth/gemma-3-1b-it, gemma3-1b, false, nvfp4) / test-mlx-llm-gemma3-1b-nvfp4 (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 2
• MLX / test-mlx-llm (unsloth/gemma-3-1b-it, gemma3-1b, true, 4w) / test-mlx-llm-gemma3-1b-custom-4w (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 2
• MLX / test-mlx-llm (unsloth/gemma-3-1b-it, gemma3-1b, true, nvfp4) / test-mlx-llm-gemma3-1b-custom-nvfp4 (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 2
• MLX / test-mlx-llm (unsloth/Llama-3.2-1B-Instruct, llama-1b, false, 4w) / test-mlx-llm-llama-1b-4w (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 2
• MLX / test-mlx-llm (unsloth/Llama-3.2-1B-Instruct, llama-1b, false, nvfp4) / test-mlx-llm-llama-1b-nvfp4 (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 2
• MLX / test-mlx-llm (unsloth/Llama-3.2-1B-Instruct, llama-1b, true, 4w) / test-mlx-llm-llama-1b-custom-4w (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 2
• MLX / test-mlx-llm (unsloth/Llama-3.2-1B-Instruct, llama-1b, true, nvfp4) / test-mlx-llm-llama-1b-custom-nvfp4 (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 2
• MLX / test-mlx-llm (unsloth/Qwen3-0.6B, qwen3-0.6b, false, 4w) / test-mlx-llm-qwen3-0.6b-4w (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 2
• MLX / test-mlx-llm (unsloth/Qwen3-0.6B, qwen3-0.6b, false, nvfp4) / test-mlx-llm-qwen3-0.6b-nvfp4 (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 2
• MLX / test-mlx-llm (unsloth/Qwen3-0.6B, qwen3-0.6b, true, 4w) / test-mlx-llm-qwen3-0.6b-custom-4w (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 2
• MLX / test-mlx-llm (unsloth/Qwen3-0.6B, qwen3-0.6b, true, nvfp4) / test-mlx-llm-qwen3-0.6b-custom-nvfp4 (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 2
• MLX / test-mlx-whisper / test-mlx-whisper (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 2
• pull / android / run-emulator (gh)
  The process '/usr/local/lib/android/sdk/platform-tools/adb' failed with exit code 224
• pull / test-samsung-models-linux / linux-job (gh)
  test_w2l_fp16
• trunk / unittest-release / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• trunk / test-models-macos-coreml (ic4) / macos-job (gh)
  ##[error]The operation was canceled.
• trunk / test-models-macos-coreml (w2l) / macos-job (gh)
  ##[error]The operation was canceled.

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• trunk / unittest-release / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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