Suppres ptxas stack size warning in Debug mode (#1033)

PR #939 introduced CUTLASS dependency. When compiled in debug mode, this leads to the following error:

```
ptxas error   : Stack size for entry function '_ZN12raft_cutlass6KernelINS_...' cannot be statically determined
```

This would be normally just a warning, but we treat warnings as errors. This PR disables the warning in Debug mode.

Authors:
  - Tamas Bela Feher (https://github.com/tfeher)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)

URL: #1033

cpp/cmake/modules/ConfigureCUDA.cmake

@@ -47,4 +47,5 @@ endif()
 if(CMAKE_BUILD_TYPE MATCHES Debug)
   message(VERBOSE "RAFT: Building with debugging flags")
   list(APPEND RAFT_CUDA_FLAGS -G -Xcompiler=-rdynamic)
+  list(APPEND RAFT_CUDA_FLAGS -Xptxas --suppress-stack-size-warning)
 endif()