Update ExecuTorch install requirements to new APIs

[Jack-Khuu]

🚀 The feature, motivation and pitch

As a follow up from #1475, torchchat should verify whether the ExecuTorch install instructions properly reflect their new instructions.

Specifically ET:

• renamed the original ./install_requirements to ./install_executorch (installs dependencies + pip installs)
• Introduced a new ./install_requirements command that only installs the ET requirements

See pytorch/executorch#7730 for the relevant stack.

Alternatives

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Additional context

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RFC (Optional)

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[Jack-Khuu]

Calling out that this may be a no-op:

In that case justify and verify that the existing instructions are sufficient

[silverguo]

It seems that most of works were done by pytorch/executorch#7730.
For installing ET from source, torchchat is doing the correct step by calling ./install_executorch.sh after cloning ET source code.
Installing with --pybind off is currently broken and sent a PR as a fix #1509