pip install reference_model and use pybind (#7077)

* pip install reference_model and use pybind

The reference model is pip installed in setup.sh.
Also install vela similarily.
Since the installation contains serialization_lib,
we don't have to include it as a package
in Executorch's setup.py. The serialization_lib
is still needed as a submodule in the arm backend
to find the tosa.fbs for deserialization.

Change-Id: I24fff6c00a3961444de5d878ab169d5ba4c9156d

* Address CR comments

Signed-off-by: Erik Lundell <erik.lundell@arm.com>
Change-Id: If733617374683765bf1d49ff8f64e4c7ab9bc42d

---------

Signed-off-by: Erik Lundell <erik.lundell@arm.com>