Add sanity checks when GPU is requested

[antoniupop]

Ensure that the compiler/runtime have GPU capability and that at least one device is available to run on.
If the compiler/runtime was not compiled with Cuda enabled (e.g., a non-GPU wheel) and GPU support is requested (use_gpu) emit warning message and continue without GPU, using CPU only with loop parallelism.
If the compiler or wheel are GPU capable, but no GPU is found, emit warning and continue on CPU.

[bcm-at-zama]

Asking for a friend: it would also be nice, at the frontend level, to be sure there is a GPU available, when people want to run in --use_gpu. Such that CML can return a clean error if users ask GPU execution without the wheel or the HW

[antoniupop]

Asking for a friend: it would also be nice, at the frontend level, to be sure there is a GPU available, when people want to run in --use_gpu. Such that CML can return a clean error if users ask GPU execution without the wheel or the HW

So this can be made into an error, but now it's a warning. It can be brought further up, but it's already covered with this PR. If the wheel is not GPU capable you get:
This instance of the Concrete compiler does not support GPU acceleration. If you are using Concrete-Python, it means that the module installed is not GPU enabled. Continuing without GPU acceleration.
If the HW is not present or available:
No Cuda device available on this system (either not present or the driver is not online). Continuing without GPU acceleration.

[bcm-at-zama]

I don't know exactly how it works @antoniupop but CML will need to be able to try/catch the error I think, so could you do something that they can intercept, please? Maybe it's already the case, just asking

[bcm-at-zama]

Maybe making a CP function is_GPU_capable() or something like that, that they can easily call?

[antoniupop]

I don't know exactly how it works @antoniupop but CML will need to be able to try/catch the error I think, so could you do something that they can intercept, please? Maybe it's already the case, just asking

It's not an exception, just a warning. If CML wants to test at their level, it's just a matter of querrying the Cuda runtime, but we can also provide a compiler level function to check.

[antoniupop]

Maybe making a CP function is_GPU_capable() or something like that, that they can easily call?

Yes that would be the most reasonable. There is a function already, but only covers HW, so could add compiler configuration too.

[bcm-at-zama]

Yes better to have a function in CP, for direct users of CP or for CML. Thank you

[antoniupop]

Yes better to have a function in CP, for direct users of CP or for CML. Thank you

So I've added two functions to CP, concrete.compiler.check_gpu_enabled() and concrete.compiler.check_gpu_available() to check if the compiler is GPU capable and if a GPU can be used. I'll have to test these separately as the CI won't be able to assess this.