[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/execution/test_on.cu

@@ -0,0 +1,125 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of CUDA Experimental 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) 2024 NVIDIA CORPORATION & AFFILIATES.
+//
+//===----------------------------------------------------------------------===//
+
+#include <cuda/experimental/execution.cuh>
+
+#include "testing.cuh"
+
+namespace ex = cudax::execution;
+
+__host__ __device__ bool _on_device() noexcept
+{
+  NV_IF_ELSE_TARGET(NV_IS_HOST, //
+                    ({ return false; }),
+                    ({ return true; }));
+}
+
+auto const main_thread_id = ::std::this_thread::get_id();
+
+void simple_start_on_thread_test()
+{
+  ex::thread_context ctx;
+  auto sch  = ctx.get_scheduler();
+  auto sndr = ex::on(sch, ex::just() | ex::then([] {
+                            CHECK(::std::this_thread::get_id() != main_thread_id);
+                          }))
+            | ex::then([]() -> int {
+                CHECK(::std::this_thread::get_id() == main_thread_id);
+                return 42;
+              });
+  auto [result] = ex::sync_wait(std::move(sndr)).value();
+  CHECK(result == 42);
+}
+
+void simple_continue_on_thread_test()
+{
+  ex::thread_context ctx;
+  auto sch  = ctx.get_scheduler();
+  auto sndr = ex::just() | ex::on(sch, ex::then([] {
+                                    CHECK(::std::this_thread::get_id() != main_thread_id);
+                                  }))
+            | ex::then([]() -> int {
+                CHECK(::std::this_thread::get_id() == main_thread_id);
+                return 42;
+              });
+  auto [result] = ex::sync_wait(std::move(sndr)).value();
+  CHECK(result == 42);
+}
+
+void simple_start_on_stream_test()
+{
+  cudax::stream str{cuda::device_ref(0)};
+  auto sch  = cudax::stream_ref{str};
+  auto sndr = ex::on(sch, ex::just(42) | ex::then([] __host__ __device__(int i) noexcept -> int {
+                            return _on_device() ? i : -i;
+                          }))
+            | ex::then([] __host__ __device__(int i) noexcept -> int {
+                return _on_device() ? -1 : i;
+              });
+  auto [result] = ex::sync_wait(std::move(sndr)).value();
+  CHECK(result == 42);
+}
+
+void simple_continue_on_stream_test()
+{
+  cudax::stream str{cuda::device_ref(0)};
+  auto sch  = cudax::stream_ref{str};
+  auto sndr = ex::just(42) | ex::on(sch, ex::then([] __host__ __device__(int i) noexcept -> int {
+                                      return _on_device() ? i : -i;
+                                    }))
+            | ex::then([] __host__ __device__(int i) noexcept -> int {
+                return _on_device() ? -1 : i;
+              });
+  auto [result] = ex::sync_wait(std::move(sndr)).value();
+  CHECK(result == 42);
+}
+
+void test_continues_on_updates_env()
+{
+  ex::thread_context ctx;
+  auto sch  = ctx.get_scheduler();
+  auto sndr = ex::just() | ex::on(sch, ex::let_value([] {
+                                    return ex::read_env(ex::get_scheduler);
+                                  }))
+            | ex::then([](auto sch2) -> int {
+                STATIC_REQUIRE(cuda::std::same_as<decltype(sch2), decltype(sch)>);
+                return 42;
+              });
+  auto [result] = ex::sync_wait(std::move(sndr)).value();
+  CHECK(result == 42);
+}
+
+namespace
+{
+C2H_TEST("simple on(sch, sndr) thread test", "[on]")
+{
+  simple_start_on_thread_test();
+}
+
+C2H_TEST("simple on(sndr, sch, closure) thread test", "[on]")
+{
+  simple_continue_on_thread_test();
+}
+
+C2H_TEST("simple on(sch, sndr) stream test", "[on][stream]")
+{
+  simple_start_on_stream_test();
+}
+
+C2H_TEST("simple on(sndr, sch, closure) stream test", "[on][stream]")
+{
+  simple_continue_on_stream_test();
+}
+
+C2H_TEST("test that on(sndr, sch, closure) updates the env for closure", "[on][stream]")
+{
+  test_continues_on_updates_env();
+}
+} // namespace