Make custom_ops import optional in native runners and improve error message

[Copilot]

Summary

Running native Python runners (e.g., Qwen 3.5 examples) fails with a cryptic AssertionError: Expected 1 library but got 0 when custom_ops_aot_lib is not built. The custom ops Meta kernels registered by this import are only needed for export tracing, not for pybindings inference.

• extension/llm/custom_ops/custom_ops.py: Improve assertion message to include the search path and actionable guidance (-DEXECUTORCH_BUILD_KERNELS_LLM_AOT=ON or pybind preset).
• examples/models/llama/runner/native.py, examples/models/llama3_2_vision/runner/native.py: Wrap custom_ops import in try/except — follows the same pattern as kernels/portable/__init__.py and kernels/quantized/__init__.py.

Test plan

Verified the native runner no longer crashes on import when custom_ops_aot_lib is not built. When custom_ops is needed (e.g., during export) and the library is missing, the error now reads:

AssertionError: Expected 1 custom_ops_aot_lib library but got 0 (searched in .../extension/llm/custom_ops).
If building from source, re-build with -DEXECUTORCH_BUILD_KERNELS_LLM_AOT=ON or use the pybind cmake preset.

[pytorch-bot]

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