Get rid of fixed number cmake --build -j in sh and docs

[kirklandsign]

📚 The doc issue

Right now we have a few places where the number of cmake --build -j is fixed.
https://github.com/search?q=repo%3Apytorch%2Fexecutorch+%22+-j%22&type=code

Ideally let's use
mac: $(sysctl -n hw.ncpu) - 1
linux: $(nproc) - 1

Update these docs and scripts.

Suggest a potential alternative/fix

No response

cc @larryliu0820 @jathu

[MichaelIspas]

I would like to be assigned this issue if possible, thank you.

[larryliu0820]

I would like to be assigned this issue if possible, thank you.

Feel free to take it and submit PR.

[MichaelIspas]

Thank you @larryliu0820, I've created a pull request #11351