* onnx: enable external weights to enable Caffe2 support
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnx: update ONNX importer interface documentation
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnx: after review updates
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>