Take very long time to build torch-sys with CUDA about tch-rs

[LaurentMazare]

Thanks for reporting this.
First I would strongly recommend you to install torch manually and set the LIBTORCH environment variable. This is described in the main readme and will avoid having to download/unzip this file on each cargo clean.

Concerning the zip performance, my guess is that this is caused by the build script being compiled in debug mode. Hopefully this rfc will let you compile the build script in release mode even when compiling overall in debug mode.
An easy thing that we could tweak now would be on linux platforms to first try running unzip -d fallback to the current behavior if this fails.

[jerry73204]

I missed that. Thanks. I think we could elaborate that LIBTORCH env indicates the install prefix in README. I thought it was /usr/include/torch or some, but found be /usr when I read the source code.
Cargo profile dependencies works for me. Though it's available when rust nightly is default., it reduces time to handful minutes.