Use the 21.08 branch of rapids-cmake as rmm requires it

[robertmaynard]

Now that rmm uses rapids-cmake we need to update to the 21.08 branch to get the new rapids_cmake_write_version_file function

[dantegd]

Before RMM's migration to rapids-cmake RAFT's CMake was just relying on RMM's for all of thrust's build needs, but now this is running into

CMake Error at /opt/conda/envs/rapids/lib/cmake/rmm/rmm-config.cmake:122 (thrust_create_target):
  Unknown CMake command "thrust_create_target".

Does that mean that we need a get_thrust in raft (like cuML's)?

[robertmaynard]

Before RMM's migration to rapids-cmake RAFT's CMake was just relying on RMM's for all of thrust's build needs, but now this is running into

CMake Error at /opt/conda/envs/rapids/lib/cmake/rmm/rmm-config.cmake:122 (thrust_create_target):
  Unknown CMake command "thrust_create_target".

Does that mean that we need a get_thrust in raft (like cuML's)?

Yes that is the easiest way to make sure we pull a version of Thrust of the sufficient version that has that CMake function. I have updated this PR to do this.

[dantegd]

@gpucibot merge