Overhaul Nix Infrastructure

[collinarnett]

This PR removes the dependecy on haskell.nix and instead relies on the vanilla haskell infrastructure for nix. The primary means of exporting hasktorch will now be an overlay.

Merge Criteria:

• Support for GPU
• Support for
  • codegen
  • hasktorch
  • libtorch-ffi
  • libtorch-ffi-helper
  • hasktorch-gradually-typed
• Update Nix CI
• Update README with new Nix instructions

[collinarnett]

Waiting on NixOS/nixpkgs#306749 to be merged for torch 2.3 support.

[junjihashimoto]

@collinarnett libtorch 2.3 of nixpkgs is available.

[collinarnett]

OK I'll work on getting this compatible ASAP.

[collinarnett]

Now all packages build with nix flake check .. I still need to run and package the tests on CPU and CUDA.

[sidkshatriya]

Very fantastic to see everything using vanilla nix. So many lines of custom nix code removed !

I'm eagerly awaiting the merge of this great PR. Looks like its basically complete right ?

P.S. I looked at this in more detail -- this is still missing ability to nix develop and so on

[collinarnett]

@sidkshatriya

Looks like its basically complete right ?

Yes from a functional perspective it's pretty much finished. You can try it out by using my branch as a flake input and then adding the overlay to your flake.

this is still missing ability to nix develop and so on

At the moment the priority is distributing hasktorch but since this PR uses the default nix haskell infra it's probably safe to assume making a development shell is a simple as using shellFor with the outputs from the overlay.

Either way, I plan on updating https://github.com/collinarnett/hasktorch-skeleton this weekend to give people a demo project of how hasktorch is used as well as packaging the tests to finish up this PR.

[collinarnett]

@sidkshatriya I've gone ahead and updated hasktorch-skeleton. So go ahead and play around with it and let me know if you have any feedback. It may take a while to compile all the needed packages but once everything is tested a bit more, I'll figure out how to update the CI to get cachix setup.