GPU architecture sm_70 is at end of support

[teor2345]

Recent versions of nvcc (12.8.93) warn during compilation:

warning: subspace-proof-of-space-gpu@0.1.0: nvcc warning : Support for offline compilation for architectures prior to '<compute/sm/lto>_75' will be removed in a future release (Use -Wno-deprecated-gpu-targets to suppress warning).

Currently we declare support for sm_70:

subspace/shared/subspace-proof-of-space-gpu/build.rs

Line 73 in f55a2a3

nvcc.flag("-arch=sm_70");

Background

JIT compilation will be available in the legacy driver, but precompilation won't:
https://www.phoronix.com/news/Maxwe--Pascal-Volta-Legacy-Near

This impacts the Maxwell, Pascal, and Volta generations of GPUs:

• GeForce GTX 750 series released in 2014
• GTX 900 series
• GeForce GTX 1000 series
• Tesla V100 / DGX-1 and Titan V released in 2017

[nazar-pc]

Is when compiling on the host CUDA version?

Because we specifically use older version in Dockerfile so we support corresponding older driver versions on Linux (there is a driver version dependency in CUDA runtime version).

We do not care about anything older than Volta because Volta/Turing are the oldest architectures supported anyway (primarily due to the small amount of shared cache on older generations). Don't think removing Volta support is a huge issue with those cards being primarily professional ones, but at the same time I don't see a strong compelling reason to do so either.