Add missing model tracing step to cpp example readme, fixes pytorch#7952 (pytorch#7963)

Co-authored-by: Philip Meier <github.pmeier@posteo.de>

Author: abstractionmage

examples/cpp/hello_world/README.rst

@@ -10,6 +10,7 @@ Once both dependencies are sorted, we can start the CMake fun:
 
 1) Create a ``build`` directory inside the current one.
 2) from within the ``build`` directory, run the following commands:
+    - ``python ../trace_model.py`` To use a torchvision model in C++, you must first export it from the python version of torchvision. More information can be found on the corresponding `documentation page <https://pytorch.org/tutorials/advanced/cpp_export.html#loading-a-torchscript-model-in-c>`_.
     - | ``cmake -DCMAKE_PREFIX_PATH="<PATH_TO_LIBTORCH>;<PATH_TO_TORCHVISION>" ..``
       | where ``<PATH_TO_LIBTORCH>`` and ``<PATH_TO_TORCHVISION>`` are the paths to the libtorch and torchvision installations.
     - ``cmake --build .``