fix cuda extension on julia 1.12

[CarloLucibello]

Fix #2625 while maintaining retrocompatibility, that is

using CUDA, Flux

is all you need for CUDA support, provided cuDNN is available in your environment.

This solution is still not orthodox, so I'm not sure it won't have bad interactions with other package extensions.

The other possibilities are:

• Recommended to use using CUDA, cuDNN, Flux
• Introduce an auxiliary package (FluxCUDA) requiring CUDA and cuDNN, so that we can do using FluxCUDA, Flux.

None of these 2 options would allow old code to keep working on julia 1.12 as this PR instead does.

See also JuliaLang/julia#60307

[CarloLucibello]

@potaslab would you like to give this a try?

[mcabbott]

Thanks Carlo!

Sorry I meant to actually try, but if this try block works then it's very close in spirit to what we had, so seems like the right thing to do.

[CarloLucibello]

I will do some more testing before tagging, but so far it seems identical to previous behavior