GPU Rules and tests

[willtebbutt]

Places to look for rules and tests for GPU stuff:

• Zygote.jl tests
• Tracker.jl tests: no GPU tests found.
• Lux.jl integration test (here, needs here)
• NNlib.jl: current unit tests, potentially more
• Flux.jl unit tests

@yebai anything else that you can think of that we should be looking at?