Use target_compile_definitions instead of add_definitions when setting ONNX_NAMESPACE (#1318)
Use target_compile_definitions instead of add_definitions when setting ONNX_NAMESPACE (#1318)

* Use target_compile_options instead of add_definitions

* Set compile flags about namespace for onnx_cpp2py_export

* Use target_compile_definitions instead of add_definitions for ONNX_ML

* Use target_compile_definitions instead of target_compile_options

onnx/onnx,onnx/onnx,onnx/onnx,onnx/onnx