At some point pytorch.dev was added to expose the libtorch headers and
libraries to non-Python users of libtorch. However, this output
currently has two disadvantages:
1. An application that compiles against the dev output will also have
the libtorch header files in its closure. This is not so nice when
e.g. building Docker images of applications that use libtorch.
2. The dev output has a large transitive closure with many dependencies
that are not necessary when compiling against libtorch.
This change adds the `lib` output so that applications that only link
against libtorch libraries have a small closure.
Before this change, the libtorch dependency adds 746MiB:
% nix path-info -S `realpath result-dev`
/nix/store/10rmy81bjk628sfpbj2szxlws6brq1xn-python3.8-pytorch-1.5.1-dev 782203848
With this change it is reduced to 196MiB:
% nix path-info -S `realpath result-lib`
/nix/store/bck65lf0z7gdhcf89w1zs5nz333lhgwa-python3.8-pytorch-1.5.1-lib 205865056
2611e96bf3