Make examples/models/llama2/install_requirements.sh executable (#2833)

Summary:
Pull Request resolved: #2833

I had to do this `chmod 755 install_requirements.sh`

Otherwise, I have to manually chmod it when reproducing the instructions.

Reviewed By: dbort

Differential Revision: D55706006

fbshipit-source-id: 891b6e0f52256fcef843b701393a50c9b20a62d7

Author: mergennachin