Custom execution provider

onnx_light_ort/custom_ep/ reserves the independent ORT-loadable EP library. There is no implementation, CMake target or executable unit test yet; the directory README states that limitation explicitly.

The future library owns CreateEpFactories, ReleaseEpFactory, factory and device callbacks, node selection and kernel-registry or compiled-partition callbacks. It will use EP bridge: onnx-light called by ORT and its Shared support for onnx-light in ORT dependency, not Custom ops or the ops bridge. It must never link Shared support for hosting ORT-compatible plugins or the incoming bridges, or interpose their compatibility API tables. The first EP can support selected standard ONNX nodes without rewriting their domain/name, leaving unsupported nodes to the real ORT host.

Required tests with the first implementation

Factory/device ownership, partial-registration failure, capability selection, unsupported-node fallback, kernel-state reuse, changing shapes and teardown must be unit-tested when their code exists. A separate real-ORT test must run a mixed supported/unsupported graph and verify numerical output and placement. Testing empty declarations now would not validate those behaviors.

See Standalone ORT plugin interoperability for the public callback inventory, CPU-first milestones and the later compiling-EP extension.