Update install_requirements to install --pybind xnnpack by default. (#7473)

Summary:

Regular ./install_requirements.sh will install pybind xnnpack by default. It is necessary for Llama for instance.

It is still backwards compatible with './install_requirements.sh --pybind xnnpack'

One can also turn off explicitly by calling './install_requirement.sh --pybind off'

Test Plan:

Test valid options:

./install_requirements.sh
./install_requirements.sh --pybind xnnpack
./install_requirements.sh --pybind coreml
./install_requirements.sh --pybind coreml xnnpack
./install_requirements.sh --pybind off

Invalid options:

./install_requirements.sh xnnpack
./install_requirements.sh --pybind coreml off
./install_requirements.sh --pybind coreml xnnpack off
./install_requirements.sh off