Drop support for CUDA legacy stream.

CMakeLists.txt

@@ -71,7 +71,6 @@ option(HIDE_CXX_SYMBOLS "Build shared library and hide all C++ symbols" OFF)
 option(KEEP_BUILD_ARTIFACTS_IN_BINARY_DIR "Output build artifacts in CMake binary dir" OFF)
 ## CUDA
 option(USE_CUDA  "Build with GPU acceleration" OFF)
-option(USE_PER_THREAD_DEFAULT_STREAM "Build with per-thread default stream" ON)
 option(USE_NCCL  "Build with NCCL to enable distributed GPU support." OFF)
 # This is specifically designed for PyPI binary release and should be disabled for most of the cases.
 option(USE_DLOPEN_NCCL "Whether to load nccl dynamically." OFF)

cmake/Utils.cmake

@@ -80,12 +80,8 @@ function(xgboost_set_cuda_flags target)
     $<$<COMPILE_LANGUAGE:CUDA>:--expt-extended-lambda>
     $<$<COMPILE_LANGUAGE:CUDA>:--expt-relaxed-constexpr>
     $<$<COMPILE_LANGUAGE:CUDA>:-Xcompiler=${OpenMP_CXX_FLAGS}>
-    $<$<COMPILE_LANGUAGE:CUDA>:-Xfatbin=-compress-all>)
-
-  if(USE_PER_THREAD_DEFAULT_STREAM)
-    target_compile_options(${target} PRIVATE
-            $<$<COMPILE_LANGUAGE:CUDA>:--default-stream per-thread>)
-  endif()
+    $<$<COMPILE_LANGUAGE:CUDA>:-Xfatbin=-compress-all>
+    $<$<COMPILE_LANGUAGE:CUDA>:--default-stream per-thread>)
 
   if(FORCE_COLORED_OUTPUT)
     if(FORCE_COLORED_OUTPUT AND (CMAKE_GENERATOR STREQUAL "Ninja") AND