flakeify

[stites]

Changes:

• add flake support
• pytorch-world -> libtorch-nix
• add binary (for showcasing how this is hooked into a flake).

FIXMEs:

• I just hardcoded cudaSupport = false in the flake. probably I should figure out how to pass arguments in a flake-based project.
• the flake.lock should probably be replaced with something cached upstream -- I'm just reusing an lock from a different project. I started off reusing commits from niv but something went wrong and I wasn't pulling from cache. everything is still looking good on the niv setup, though.

[junjihashimoto]

Awesome! I don't think we should use the upstream cache. I hope the result of this skeleton will be cached with CUDA.

[junjihashimoto]

I want to use this pr, so I'll merge it.

[junjihashimoto]

The previous way of using default.nix is ​​not changed.