Fix CMake Defaults

[CascadingRadium]

• If optimization level is not specified, force usage of "generic"

[Copilot]

Pull Request Overview

This PR fixes the incorrect declaration of FAISS_OPT_LEVEL in the CMake configuration. Previously, it was defined using option() which is meant for boolean values, but it should accept string values for different optimization levels (generic, avx2, avx512, etc.). The fix replaces the option() with a proper conditional string cache variable.

• Changed FAISS_OPT_LEVEL from an option() to a CACHE STRING variable
• Added conditional check to only set the variable if not already defined
• Duplicated the comment line explaining valid values

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.