Use hasktorch in nixpkgs

[junjihashimoto]

This PR improves the workflow by using hasktorch in nixpkgs and installing it with ghc, to avoid long builds of hasktorch.

I uploaded hasktorch to hackage so that we can use hasktorch from nixpkgs.
Building the skeleton is simplified to:

[hasktorch-skeleton (feature/nixpkgs-hasktorch)]$ nix develop
[hasktorch-skeleton (feature/nixpkgs-hasktorch)]$ cabal build all
Configuration is affected by the following files:
- cabal.project
Warning: The package list for 'hackage.haskell.org' is 34 days old.
Run 'cabal update' to get the latest list of available packages.
Resolving dependencies...
Build profile: -w ghc-9.6.6 -O1
In order, the following will be built (use -v for more details):
 - hasktorch-skeleton-0.0.0.0 (lib) (first run)
 - hasktorch-skeleton-0.0.0.0 (exe:example) (first run)
Configuring library for hasktorch-skeleton-0.0.0.0...
Preprocessing library for hasktorch-skeleton-0.0.0.0...
Building library for hasktorch-skeleton-0.0.0.0...
[1 of 1] Compiling HasktorchSkeleton ( src/HasktorchSkeleton.hs, dist-newstyle/build/x86_64-linux/ghc-9.6.6/hasktorch-skeleton-0.0.0.0/build/HasktorchSkeleton.o, dist-newstyle/build/x86_64-linux/ghc-9.6.6/hasktorch-skeleton-0.0.0.0/build/HasktorchSkeleton.dyn_o )
Configuring executable 'example' for hasktorch-skeleton-0.0.0.0...
Preprocessing executable 'example' for hasktorch-skeleton-0.0.0.0...
Building executable 'example' for hasktorch-skeleton-0.0.0.0...
[1 of 1] Compiling Main             ( exe/Main.hs, dist-newstyle/build/x86_64-linux/ghc-9.6.6/hasktorch-skeleton-0.0.0.0/x/example/build/example/example-tmp/Main.o )

exe/Main.hs:3:1: warning: [-Wunused-imports]
    The import of ‘HasktorchSkeleton’ is redundant
      except perhaps to import instances from ‘HasktorchSkeleton’
    To import instances alone, use: import HasktorchSkeleton()
  |
3 | import HasktorchSkeleton
  | ^^^^^^^^^^^^^^^^^^^^^^^^
[2 of 2] Linking dist-newstyle/build/x86_64-linux/ghc-9.6.6/hasktorch-skeleton-0.0.0.0/x/example/build/example/example

[collinarnett]

Is libtorch-bin needed here? It's already linked in libtorch-ffi from your other PR https://github.com/junjihashimoto/nixpkgs/blob/0145fac03b41ddc9ef961d507af19b6ca0f93268/pkgs/development/haskell-modules/configuration-nix.nix#L1526

[junjihashimoto]

@collinarnett Thank you for your review!
This is the same as the one referenced by libtorch-ffi and is not needed.
It is necessary when creating custom operators in C++, but I will remove it from here.

[collinarnett]

I think most developers have their own IDE so overriding the path to that IDE here would not be good.

[junjihashimoto]

We definitely don't need vscode.
Even if we develop in the environment, it's not necessary when using remote-ssh from vscode on the local macos.