Simplify pip install step of install_requirements.sh (#2731)

Summary:
It's not clear why we were using --force-reinstall; if the requested versions are different from what's currently installed, pip will handle the reinstallation.

And calling pip multiple times with --force-reinstall meant that we were uninstalling and reinstalling the same packages over and over.

Now we only call `pip install` once for requirements, and we do not force reinstall. On my machine, without enabling pybindings, a no-op run of this script (i.e. when all packages are up-to-date) takes 7 seconds, including the `pip install .`.

Pull Request resolved: https://github.com/pytorch/executorch/pull/2731

Test Plan:
```
./install_requirements.sh
```

Reviewed By: mergennachin, kirklandsign

Differential Revision: D55456845

Pulled By: dbort

fbshipit-source-id: 3f5cb77ab86f219fdd12e985f26b2817604ce5cb