[FIX] Get libcudacxx from cuco

[trxcllnt]

This PR fixes a CMake configuration failure when using RAFT's build dir as the package root, e.g.:

cmake -S /cuml/cpp -B /cuml/cpp/build -Draft_ROOT=/raft/cpp/build

CMake fails with this mysterious error:

  The link interface of target "raft::raft" contains:

    std::mdspan

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

However the problem isn't mdspan.

The error occurs when raft-dependencies.cmake tries and fails to find libcudacxx:

raft/cpp/build/release/raft-dependencies.cmake(206):  find_dependency(libcudacxx )
conda/cuda_11.6/envs/rapids/share/cmake-3.23/Modules/CMakeFindDependencyMacro.cmake(34):  set(cmake_fd_quiet_arg )
conda/cuda_11.6/envs/rapids/share/cmake-3.23/Modules/CMakeFindDependencyMacro.cmake(35):  if(raft_FIND_QUIETLY )
conda/cuda_11.6/envs/rapids/share/cmake-3.23/Modules/CMakeFindDependencyMacro.cmake(36):  set(cmake_fd_quiet_arg QUIET )
conda/cuda_11.6/envs/rapids/share/cmake-3.23/Modules/CMakeFindDependencyMacro.cmake(38):  set(cmake_fd_required_arg )
conda/cuda_11.6/envs/rapids/share/cmake-3.23/Modules/CMakeFindDependencyMacro.cmake(39):  if(raft_FIND_REQUIRED )
conda/cuda_11.6/envs/rapids/share/cmake-3.23/Modules/CMakeFindDependencyMacro.cmake(43):  get_property(cmake_fd_alreadyTransitive GLOBAL PROPERTY _CMAKE_libcudacxx_TRANSITIVE_DEPENDENCY )
conda/cuda_11.6/envs/rapids/share/cmake-3.23/Modules/CMakeFindDependencyMacro.cmake(47):  find_package(libcudacxx  QUIET  )
CMake Debug Log at conda/cuda_11.6/envs/rapids/share/cmake-3.23/Modules/CMakeFindDependencyMacro.cmake:47 (find_package):
  find_package considered the following paths for Findlibcudacxx.cmake:

    cuml/cpp/build/cuda-11.6.0/branch-22.06/release/CPM_modules/Findlibcudacxx.cmake
    cuml/cpp/build/cuda-11.6.0/branch-22.06/release/_deps/rapids-cmake-src/rapids-cmake/Findlibcudacxx.cmake
    conda/cuda_11.6/envs/rapids/share/cmake-3.23/Modules/Findlibcudacxx.cmake

  The file was not found.

  _ROOT CMake variable [CMAKE_FIND_USE_PACKAGE_ROOT_PATH].

    raft/cpp/build/release

  CMAKE_PREFIX_PATH variable [CMAKE_FIND_USE_CMAKE_PATH].

    conda/cuda_11.6/envs/rapids

  CMAKE_FRAMEWORK_PATH and CMAKE_APPBUNDLE_PATH variables
  [CMAKE_FIND_USE_CMAKE_PATH].

  Env variable libcudacxx_DIR [CMAKE_FIND_USE_CMAKE_ENVIRONMENT_PATH].

    none

  CMAKE_PREFIX_PATH env variable [CMAKE_FIND_USE_CMAKE_ENVIRONMENT_PATH].

    none

  CMAKE_FRAMEWORK_PATH and CMAKE_APPBUNDLE_PATH env variables
  [CMAKE_FIND_USE_CMAKE_ENVIRONMENT_PATH].

    none

  Paths specified by the find_package HINTS option.

    none

  Standard system environment variables
  [CMAKE_FIND_USE_SYSTEM_ENVIRONMENT_PATH].

    conda/cuda_11.6
    /usr/local
    /usr
    /
    /usr/local/cuda

  CMake User Package Registry [CMAKE_FIND_USE_PACKAGE_REGISTRY].

    none

  CMake variables defined in the Platform file
  [CMAKE_FIND_USE_CMAKE_SYSTEM_PATH].

    /usr/X11R6
    /usr/pkg
    /opt

  CMake System Package Registry
  [CMAKE_FIND_PACKAGE_NO_SYSTEM_PACKAGE_REGISTRY].

    none

  Paths specified by the find_package PATHS option.

    none

  find_package considered the following locations for libcudacxx's Config
  module:

    raft/cpp/build/release/libcudacxxConfig.cmake
    raft/cpp/build/release/libcudacxx-config.cmake
    raft/cpp/build/release/cmake/libcudacxxConfig.cmake
    raft/cpp/build/release/cmake/libcudacxx-config.cmake
    conda/cuda_11.6/envs/rapids/libcudacxxConfig.cmake
    conda/cuda_11.6/envs/rapids/libcudacxx-config.cmake
    conda/cuda_11.6/libcudacxxConfig.cmake
    conda/cuda_11.6/libcudacxx-config.cmake
    /usr/local/libcudacxxConfig.cmake
    /usr/local/libcudacxx-config.cmake
    /usr/libcudacxxConfig.cmake
    /usr/libcudacxx-config.cmake
    /libcudacxxConfig.cmake
    /libcudacxx-config.cmake
    /usr/local/cuda/libcudacxxConfig.cmake
    /usr/local/cuda/libcudacxx-config.cmake
    /opt/libcudacxxConfig.cmake
    /opt/libcudacxx-config.cmake

  The file was not found.

Call Stack (most recent call first):
  raft/cpp/build/release/raft-dependencies.cmake:206 (find_dependency)
  raft/cpp/build/release/raft-config.cmake:85 (include)
  build/cuda-11.6.0/branch-22.06/release/cmake/CPM_0.35.0.cmake:215 (find_package)
  build/cuda-11.6.0/branch-22.06/release/cmake/CPM_0.35.0.cmake:272 (cpm_find_package)
  build/cuda-11.6.0/branch-22.06/release/_deps/rapids-cmake-src/rapids-cmake/cpm/find.cmake:152 (CPMFindPackage)
  cmake/thirdparty/get_raft.cmake:50 (rapids_cpm_find)
  cmake/thirdparty/get_raft.cmake:80 (find_and_configure_raft)
  CMakeLists.txt:200 (include)

The cause of the failure is this:

1. RAFT's CMakeLists.txt adds cuco before adding libcudacxx (here)
2. cuco is added to the raft-distance-exports export set (here)
   • cuco adds libcudacxx, because it was declared before libcudacxx in step 1:

   -- CPM: cuco: adding package libcudacxx@1.7.0 (1.7.0)
   

3. libcudacxx is added to the raft-exports export set (here)
   • Since libcudacxx has already been added by cuco in step 2, a find_dependency(libcudacxx) call (not a CPMFindPackage call) is recorded in raft-dependencies.cmake

When someone uses the raft build dir as the package root, CMake runs raft-dependencies.cmake before raft-distance-dependencies.cmake. raft-dependencies.cmake fails at find_dependency(libcudacxx), because the CPMFindPackage("NAME;cuco;...) call that is expected to introduce libcudacxx hasn't been executed yet.

Alternative fix

There are two other possible fixes aside from the one in this PR:

1. Swap the order of these two lines in CMakeLists.txt.
2. Change the libcudacxx BUILD_EXPORT_SET and INSTALL_EXPORT_SET to raft-distance-exports so find_dependency(libcudacxx) is recorded in raft-distance-dependencies.cmake after cuco (and cuco's libcudacxx dependency) has been found.

I didn't see any references to <cuda/std/...> in RAFT itself, so the cleanest option is to delete RAFT's get_libcudacxx.cmake.

[robertmaynard]

I didn't see any references to <cuda/std/...> in RAFT itself, so the cleanest option is to delete RAFT's get_libcudacxx.cmake, since it doesn't seem to be needed.

I also can't see any <cuda/std/> includes so I agree that this simplification is the best approach.

[cjnolet]

rerun tests

[cjnolet]

@gpucibot merge