Merge 3 existing PRs for OnnxDiscrepancyCheck + llama.cpp integration (with dedicated GGUF conversion pass)

[Copilot]

Describe your changes

Merges #2536, #2535, #2534.

Additionally adds llama.cpp integration and other improvements to OnnxDiscrepancyCheck and test-mode workflow handling:

• New llama_cpp flag (bool, default False) on OnnxDiscrepancyCheck — when enabled, compares inference with llama.cpp.
• New llama_cpp_env_path parameter (Optional[str]) — path to the llama_env virtual environment where llama-cpp-python and convert_hf_to_gguf.py are installed (defaults to "llama_env" relative to cwd).
• New --test_llama_path CLI option — specifies the path to the llama_env virtual environment when running with --test. Using --test_llama_path without --test emits a warning.
• New ConvertHfToGGUF pass (olive/passes/pytorch/convert_hf_to_gguf.py) — injected when --test_llama_path is provided. This pass converts the test HF model to GGUF ahead of discrepancy checking and stores the GGUF path in model attributes for downstream reuse.
• compare_llama_cpp() updates — now reuses a preconverted GGUF when available; otherwise it falls back to in-method HF→GGUF conversion. llama.cpp comparison failures are captured in discrepancy results (status/failures) instead of aborting the whole run, so ONNX generation can still complete.
• Improved --test_metrics parsing — now accepts both space-separated (--test_metrics mae speedup) and comma-separated (--test_metrics mae,speedup) forms.
• Fixed add_discrepancy_check_pass update-in-place — existing discrepancy-pass config generated by dry-run is updated in-place so current --test_metrics, --output_path, and llama settings are applied.
• Fixed test model persistence across engine cache hits — ModelBuilder stores a reference HF copy (reference_hf_model/) alongside cached ONNX outputs; discrepancy check falls back to this copy if the original test model path is missing.
• New SaveTestModelConfig pass (olive/passes/pytorch/save_test_model_config.py) — injected at the start of passes for --test; ensures test model config/marker (and random test model persistence path usage) is set up before downstream passes.
• CI workflow (test-model-fast.yml) — includes setup of a llama environment and llama.cpp conversion script dependencies.
• Updated documentation (cli-fast-test.md) — clarifies where layer reduction happens, when test-model directories are created, cache fallback behavior, and llama.cpp test flow including the dedicated GGUF conversion pass.

Checklist before requesting a review

• Add unit tests for this change.
• Make sure all tests can pass.
• Update documents if necessary.
• Lint and apply fixes to your code by running lintrunner -a
• Is this a user-facing change? If yes, give a description of this change to be included in the release notes.

(Optional) Issue link