Enabling runtime choice for mallocAsync or sync version.

[Narsil]

Initial issue: huggingface/candle#353

Basically cudarc fails on Windows even on recent cards, but also on older cards.

The core reason is that cuMemAllocAsync is not supported on those devices/OS.
This PR enables it by choosing between sync and async at runtime in the intended way by CUDA https://docs.nvidia.com/cuda/cuda-runtime-api/group__CUDART__MEMORY__POOLS.html (Check supported platform).

Alternatives:

I would have preferred to use a compile time flag for this, since it seems always preferable to use async where available and sync elsewhere.

Previous work along that line.
#106

I think we could have had a feature enabled in build.rs (rust-lang/cargo#5499) for that particular thing.
However I was unable to find an easy way to detect that capacity using nvidia-smi (or any other expected CLI).
Meaning in order to do that at compile time I think we would have to compile first cudaDeviceGetAttribute small program in order to be able to call it within build.rs. This seems way overengineered and therefore I didn't do that.

I haven't measure performance with the if everywhere.

Another variant of this PR would be to keep track of wether the Slice was initialized with Async or not, to remove the if within Drop. (We could also store it directly on CudaDevice to remote the if within alloc.)
I deemed this as a performance feature and therefore didn't do it in this PR.

[Narsil]

@coreylowman

[coreylowman]

This looks fine to me, I like the runtime detection better than adding an additional compiler flag TBH.

@Narsil it looks like this deletes some examples, can you restore those? They are probably juts from out-dated fork

[Narsil]

@coreylowman The example is actually because this examples always fails to build. I started just kicking it out whenever I'm using tests.

I'll restore for this PR, but would be nice to have a real fix.
Same goes for the 2 multi device tests they are failing on single GPU machines which does not feel super great :)

[coreylowman]

lgtm

[coreylowman]

@Narsil new version up on crates.io