Define "outside observability" in graphs spec

[EwanC]

The graph fusion proposal has had discussions about how observable the result of their optimizations is to a user intel#8678 (comment)

This comes back to whether the base graphs spec defines any outside observability requirements. We should specify this in some way for the next revision of the graphs spec.

[EwanC]

Graph fusion extension is no longer being actively work on, closing this issue. Can be reopened as an issue on intel/llvm if in future it becomes a task we'll need to track.