Downstream users have dependences on cmake variables and internals, making cmake a compatibility surface

[mikekgfb]

x-ref: https://github.com/pytorch/torchchat/issues/655

We're resolving https://github.com/pytorch/torchchat/issues/655 by pinning, but after we're in beta, downstream projects that need to build with cmake may expect their cmake files to work reliably, amking cmake and possibly internal cmake variables a compatibility surface.

We might be deliberate adn publish what users can use with a commitment to keep that interface as solid as possible, using TC to stress test (if TC can't use the defined interface, then maybe the interface is not sufficient?)