cmake: fix ONNX_NAMESPACE if USE_SYSTEM_ONNX (#54973)
Summary:
`ONNX_NAMESPACE` is empty by default if `USE_SYSTEM_ONNX ON`, while it should be equal to `onnx`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54973
Reviewed By: glaringlee
Differential Revision: D27466020
Pulled By: walterddr
fbshipit-source-id: 47cde3604acbda3f45bec5893036b39fd1eb58c9
