Support CUDA in torch-sys build script? about tch-rs

[LaurentMazare]

Sounds good! I feel that triggering it explicitly via an environment variable would be better to start with (the pre-built cuda binary will result in a large download). I think that only cuda 9.0 is supported for pre-built libtorch for now so TORCH_USE_CUDA sounds good to me.

[mwilliammyers]

I have it almost coded up using TORCH_CUDA_VERSION which checks for 9.0 or 90 and sets it to cu90, but I can also have it just check to see if that variable is set to anything and use cuda 9.0 so we can be forward compatible?

[LaurentMazare]

Asking for TORCH_CUDA_VERSION to be 9.0, 90 sounds reasonable. Maybe worth adding a comment at the top of say build.rs so that it's clear how to trigger this.

[mwilliammyers]

Ok, cool. Also, should we panic if you set TORCH_CUDA_VERSION on macOS or silently use cpu?

[LaurentMazare]

Let's panic with an appropriate error message in this case - that way things will be pretty explicit.