[INFRA] Import NVIDIA/CCCL upstream as optimization reference library

CCCL (CUDA C++ Core Libraries) provides:
- CUB: device/block/warp-level GPU primitives (reduce, scan, sort, topk)
- Thrust: high-level parallel algorithms (transform_reduce, sort, scan)
- libcudacxx: CUDA C++ standard library (atomics, barriers, memory)
- cudax: experimental features (memory resources, allocators)
- Tuning policies: per-SM hardware-specific algorithm parameters

Competition optimization vectors mapped to CCCL:
- Output TPS (83% weight): warp_reduce, block_reduce, device_topk
- Input TPS (14% weight): device_scan, block_load, prefetch
- Cache TPS (3% weight): prefix caching strategy patterns
- Memory (0.9 util): pooled/cached/buddy allocators

Source: https://github.com/NVIDIA/cccl (shallow clone, HEAD only)
License: Apache-2.0

cccl_upstream/cudax/test/stf/dot/sections_2.cu

@@ -0,0 +1,49 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of CUDASTF in CUDA C++ Core Libraries,
+// under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+// SPDX-FileCopyrightText: Copyright (c) 2022-2024 NVIDIA CORPORATION & AFFILIATES.
+//
+//===----------------------------------------------------------------------===//
+
+/**
+ * @file
+ * @brief This test makes sure we can generate a dot file with sections
+ */
+
+#include <cuda/experimental/stf.cuh>
+
+using namespace cuda::experimental::stf;
+
+int main()
+{
+// TODO (miscco): Make it work for windows
+#if !_CCCL_COMPILER(MSVC)
+  context ctx;
+  auto lA = ctx.token().set_symbol("A");
+  auto lB = ctx.token().set_symbol("B");
+  auto lC = ctx.token().set_symbol("C");
+
+  // Begin a top-level section named "foo"
+  auto s_foo = ctx.dot_section("foo");
+  for (size_t i = 0; i < 2; i++)
+  {
+    // Section named "bar" using RAII
+    auto s_bar = ctx.dot_section("bar");
+    ctx.task(lA.read(), lB.rw()).set_symbol("t1")->*[](cudaStream_t) {};
+    for (size_t j = 0; j < 2; j++)
+    {
+      // Section named "baz" using RAII
+      auto s_bar = ctx.dot_section("baz");
+      ctx.task(lA.read(), lC.rw()).set_symbol("t2")->*[](cudaStream_t) {};
+      ctx.task(lB.read(), lC.read(), lA.rw()).set_symbol("t3")->*[](cudaStream_t) {};
+      // Implicit end of section "baz"
+    }
+    // Implicit end of section "bar"
+  }
+  s_foo.end(); // Explicit end of section "foo"
+  ctx.finalize();
+#endif // !_CCCL_COMPILER(MSVC)
+}