Add section on Checking GPU Availability

[logankilpatrick]

Follow up from this question I posted: https://stackoverflow.com/questions/68142837/how-to-check-if-flux-jl-is-using-the-gpu/68144877#68144877 which was inspired by the top PyTorch question on SO: https://stackoverflow.com/questions/48152674/how-to-check-if-pytorch-is-using-the-gpu

[ToucheSir]

Depending on how long it takes for #1566 to land, we may want to pull over the default_gpu_converter functionality into Flux first and reference that here and/or add a user-facing API for checking GPU availability.

[logankilpatrick]

I don't have a good grasp of these things right now but I thought this would be useful for the time being.

[darsnack]

I think this will need to be updated for multiple GPUs, but clearer docs never hurt anyone in the meantime.

[DhairyaLGandhi]

Should we avoid talking about the Flux.use_cuda flag. Its not something stable at all, and is more of a detail which may change any time, in fact I believe Tim also suggested some changes to that mechanism.

[darsnack]

Sure we can limit this to just CUDA.functional

[logankilpatrick]

Alright I think we are at the finish line here!

[CarloLucibello]

bors r+

[bors]

Build succeeded:

• buildkite/flux-dot-jl