nccl: 2.1.4 -> 2.4.2-1

[orivej]

Motivation for this change

Since 2.3, NCCL 2 is released under BSD 3-Clause and can be built from source.

I have defined nccl_cudatoolkit_9_0, 9 (currently 9.2) and 10 because these are the builds available at https://developer.nvidia.com/nccl/nccl-download (requires a free nvidia.com account). I have removed nccl_cudatoolkit_8, because although NCCL might be buildable with CUDA 8.0, cudatoolkit_8 is broken in Nixpkgs.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip" nix-review
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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/cc @abbradar who has changed the maintainer from @hyphon81 to @mdaiter in 41abde9