Add option to limit ONNX Runtime build parallelism

[LeC911]

Motivation

When building the ONNX Runtime backend through Triton build.py, the top-level --build-parallel option limits the outer CMake build, but the generated Dockerfile.ort still invokes ONNX Runtime build as:

./build.sh --parallel

without a job count.

On resource-constrained builders this can make the ORT build fail while building libonnxruntime.so, even when Triton is built with --build-parallel=1.

Change

This PR adds an optional CMake cache variable:

TRITON_BUILD_ONNXRUNTIME_PARALLEL

When set, it is forwarded to tools/gen_ort_dockerfile.py as:

--ort-build-parallel=<N>

and the generated Dockerfile.ort uses:

./build.sh --parallel <N>

Default behavior is unchanged. If the option is not set, the generated build continues to use:

./build.sh --parallel

The original issue was reproduced on r26.04 while building Triton 2.68.0/container 26.04 with ONNX Runtime backend, CPU-only, HTTP-only, GPU disabled.

Additional validation

Verified opt-in behavior using this PR branch through Triton build.py:

--extra-backend-cmake-arg=onnxruntime:TRITON_BUILD_ONNXRUNTIME_PARALLEL=2

The generated Dockerfile.ort contains:

ARG COMMON_BUILD_ARGS="--config ${ONNXRUNTIME_BUILD_CONFIG} --parallel 2 --skip_submodule_sync --build_shared_lib ..."

Also verified default behavior remains unchanged when TRITON_BUILD_ONNXRUNTIME_PARALLEL is not set. With only the outer build parallelism set, the generated Dockerfile.ort still contains:

ARG COMMON_BUILD_ARGS="--config ${ONNXRUNTIME_BUILD_CONFIG} --parallel --skip_submodule_sync --build_shared_lib ..."