A simple install command gives everything a user need

[larryliu0820]

• We should use pip install . after pulling down all code from github. (not install_requirements.sh, at least not in official releases)
  That command by default build xnnpack and pybind.
• If a user tries to lower to other backends that are not built by default, we should give graceful error messages.

cc @mergennachin @byjlw